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9 files changed

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-67
lines changed

‎src/problems/leetcode/CombinationSum.java‎

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package problems.leetcode;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.Collections;
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import java.util.List;
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// https://leetcode.com/problems/combination-sum/
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public class CombinationSum {
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// runtime: O(2 ^ N) where N is number of candidates
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// space: O(N)
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public static List<List<Integer>> combinationSum(int[] candidates, int target) {
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if (candidates == null || candidates.length < 1) {
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return Collections.emptyList();
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}
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Arrays.sort(candidates);
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List<List<Integer>> result = new ArrayList<>();
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combinationSum(candidates, target, 0, 0, new ArrayList<>(), result);
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return result;
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}
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private static void combinationSum(int[] candidates, int target, int i, int currentSum, List<Integer> current,
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List<List<Integer>> result) {
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if (i >= candidates.length) {
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return;
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}
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int candidate = candidates[i];
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if (currentSum + candidate == target) {
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current.add(candidate);
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result.add(new ArrayList<>(current));
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current.remove(current.size() - 1);
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return;
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} else if (currentSum + candidate < target) {
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current.add(candidate);
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combinationSum(candidates, target, i, currentSum + candidate, current, result);
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current.remove(current.size() - 1);
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} else {
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return;
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}
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combinationSum(candidates, target, i + 1, currentSum, current, result);
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}
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public static void main(String[] args) {
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System.out.println(combinationSum(new int[] { 2, 3, 6, 7 }, 6));
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System.out.println(combinationSum(new int[] { 4, 2, 8 }, 8));
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}
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}

‎src/problems/leetcode/CountAndSay.java‎

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package problems.leetcode;
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// https://leetcode.com/problems/count-and-say/
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public class CountAndSay {
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// runtime: O(N)
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// space: O(N) due to recursive call stack
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public static String countAndSay(int n) {
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if (n < 2) {
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return "1";
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}
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String s = countAndSay(n - 1);
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StringBuilder sb = new StringBuilder();
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int current = s.charAt(0) - '0', count = 1;
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for (int i = 1; i < s.length(); i++) {
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int num = s.charAt(i) - '0';
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if (current == num) {
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count++;
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} else {
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sb.append(count).append(current);
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current = num;
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count = 1;
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}
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}
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sb.append(count).append(current);
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return sb.toString();
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}
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public static void main(String[] args) {
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System.out.println(countAndSay(2));
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}
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}

‎src/problems/leetcode/FirstMissingPositive.java‎

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package problems.leetcode;
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import java.util.Arrays;
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// https://leetcode.com/problems/first-missing-positive/
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public class FirstMissingPositive {
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public static int firstMissingPositive(int[] nums) {
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// https://leetcode.com/problems/first-missing-positive/solutions/17214/Java-simple-solution-with-documentation/
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// Ignore all numbers <=0 and >n since they are outside the range of possible
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// answers (which we proved was [1..n]). We do this by replacing them with the
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// value n+1.
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// For all other integers <n+1, mark their bucket (cell) to indicate the integer
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// exists. (*see below)
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// Find the first cell not marked, that is the first missing integer. If you did
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// not find an unmarked cell, there was no missing integer, so return n+1.
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int n = nums.length;
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// 1. mark numbers (num < 0) and (num > n) with a special marker number (n+1)
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// (we can ignore those because if all number are > n then we'll simply return
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// 1)
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for (int i = 0; i < n; i++) {
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if (nums[i] < 1 || nums[i] > n) {
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nums[i] = n + 1;
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}
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}
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System.out.println(Arrays.toString(nums));
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// note: all number in the array are now positive, and on the range 1..n+1
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// 2. mark each cell appearing in the array, by converting the index for that
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// number to negative
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for (int i = 0; i < n; i++) {
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int num = Math.abs(nums[i]);
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if (num > n) {
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continue;
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}
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int numIndex = num - 1;
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if (nums[numIndex] > 0) { // prevents double negative operations
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nums[numIndex] = -1 * nums[numIndex];
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}
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}
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System.out.println(Arrays.toString(nums));
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// 3. find the first cell which isn't negative (doesn't appear in the array)
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for (int i = 0; i < n; i++) {
48+
if (nums[i] >= 0) {
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return i + 1;
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}
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}
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// 4. no positive numbers were found, which means the array contains all numbers
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// 1..n
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return n + 1;
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}
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public static void main(String[] args) {
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System.out.println(firstMissingPositive(new int[] { 1, 2, 0 }));
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System.out.println(firstMissingPositive(new int[] { 3, 4, -1, 1 }));
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System.out.println(firstMissingPositive(new int[] { 7, 8, 9, 11, 12 }));
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System.out.println(firstMissingPositive(new int[] { 1, 8, 9, 11, 12 }));
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}
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}

‎src/problems/leetcode/JumpGame.java‎

Lines changed: 30 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -6,26 +6,44 @@
66
public class JumpGame {
77

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public static void main(String[] args) {
9-
// int[] nums = {2, 3, 1, 1, 4};
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int[] nums = {3, 2, 1, 0, 4};
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System.out.println(canJump(nums));
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// int[] nums = {2, 3, 1, 1, 4};
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int[] nums = { 3, 2, 1, 0, 4 };
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System.out.println(canJumpON(nums));
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}
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14-
public static boolean canJump(int[] nums) {
15-
if (nums == null) {
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// runtime: O(N)
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// space: O(1)
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public static boolean canJumpON(int[] nums) {
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for (int i = 0, boundary = 0; i <= boundary; i++) {
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boundary = Math.max(boundary, i + nums[i]);
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if (boundary >= nums.length - 1) {
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return true;
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}
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}
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return false;
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}
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// runtime: O(N^2)
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// space: O(N)
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public static boolean canJumpDP(int[] nums) {
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if (nums == null || nums.length < 1) {
1631
return false;
1732
}
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19-
int n = nums.length;
20-
boolean[] jumps = new boolean[n];
21-
jumps[n - 1] = true;
22-
for (int i = n - 2; i >= 0; i--) {
23-
for (int j = 1, maxJump = nums[i]; j <= maxJump; j++) {
24-
jumps[i] |= jumps[i + j];
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boolean[] success = new boolean[nums.length];
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int i = nums.length - 1;
36+
success[i--] = true;
37+
for (; i >= 0; i--) {
38+
for (int j = 1; j <= nums[i] && i + j < nums.length; j++) {
39+
if (success[i + j]) {
40+
success[i] = true;
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break;
42+
}
2543
}
2644
}
2745

28-
return jumps[0];
46+
return success[0];
2947
}
3048

3149
}

‎src/problems/leetcode/JumpGameII.java‎

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@@ -5,6 +5,49 @@
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*/
66
public class JumpGameII {
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// runtime: O(N^2)
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// space: O(N)
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public static int jumpON2(int[] nums) {
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int n = nums.length;
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int[] counts = new int[n];
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for (int i = n - 2; i >= 0; i--) {
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int jumps = Integer.MAX_VALUE;
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for (int j = 1; j <= nums[i] && i + j < n; j++) {
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int c = counts[i + j];
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if (c != Integer.MAX_VALUE) {
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jumps = Math.min(jumps, 1 + c);
19+
}
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}
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counts[i] = jumps;
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}
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return counts[0];
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}
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// runtime: O(N)
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// space: O(1)
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// https://leetcode.com/problems/jump-game-ii/editorial/
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public static int jumpGreedy(int[] nums) {
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// The starting range of the first jump is [0, 0]
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int answer = 0, n = nums.length;
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int curEnd = 0, curFar = 0;
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for (int i = 0; i < n - 1; ++i) {
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// Update the farthest reachable index of this jump.
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curFar = Math.max(curFar, i + nums[i]);
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40+
// If we finish the starting range of this jump,
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// Move on to the starting range of the next jump.
42+
if (i == curEnd) {
43+
answer++;
44+
curEnd = curFar;
45+
}
46+
}
47+
48+
return answer;
49+
}
50+
851
public static int jumpON(int[] nums) {
952
if (nums == null) {
1053
return 0;
@@ -32,31 +75,33 @@ public static int jumpON(int[] nums) {
3275
return jumpCount;
3376
}
3477

78+
// https://leetcode.com/problems/jump-game-ii/editorial/comments/1776566
3579
public static int jump(int[] nums) {
3680
if (nums == null) {
3781
return 0;
3882
}
3983

40-
int steps = 0;
41-
for (int l = 0, r = 0; r < nums.length - 1; steps++) {
42-
int next = r;
43-
for (int i = l; i <= r; i++) {
44-
next = Math.max(next, nums[i] + i);
84+
int steps = 0, low = 0, high = 0, n = nums.length;
85+
while (high < n - 1) {
86+
int maxJump = 0;
87+
for (int i = low; i <= high; i++) {
88+
maxJump = Math.max(maxJump, i + nums[i]);
4589
}
46-
l = r + 1;
47-
r = next;
90+
low = high + 1;
91+
high = maxJump;
92+
steps++;
4893
}
4994

5095
return steps;
5196
}
5297

5398
public static void main(String[] args) {
54-
// int[] jumps = {2, 3, 1, 1, 4};
55-
int[] jumps = {4, 1, 1, 3, 1, 1, 1};
56-
// int[] jumps = {1, 2, 1, 1, 1};
57-
// int[] jumps = {10,9,8,7,6,5,4,3,2,1,1,0};
58-
System.out.println(jump(jumps));
59-
System.out.println(jumpON(jumps));
99+
// int[] jumps = {2, 3, 1, 1, 4};
100+
int[] jumps = { 4, 1, 1, 3, 1, 1, 1 };
101+
// int[] jumps = {1, 2, 1, 1, 1};
102+
// int[] jumps = {10,9,8,7,6,5,4,3,2,1,1,0};
103+
System.out.println(jumpGreedy(jumps));
104+
// System.out.println(jumpON(jumps));
60105
}
61106

62107
}

‎src/problems/leetcode/MultiplyStrings.java‎

Lines changed: 5 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1,18 +1,18 @@
11
package problems.leetcode;
22

3+
import java.util.Arrays;
4+
35
/**
46
* https://leetcode.com/problems/multiply-strings/
57
*/
68
public class MultiplyStrings {
79

810
public static void main(String[] args) {
9-
String num1 = "12512";
10-
String num2 = "0";
11-
12-
String res = multiply(num1, num2);
13-
System.out.println(res);
11+
System.out.println(multiply("123456789", "987654321"));
1412
}
1513

14+
// runtime: O(A * B) where A and B are number of digits of the numbers
15+
// space: O(M) where M is number of digits of the greater number
1616
public static String multiply(String num1, String num2) {
1717
num1 = new StringBuilder(num1).reverse().toString();
1818
num2 = new StringBuilder(num2).reverse().toString();

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