Update readme with Zigzag Conversion solution

Added detailed explanation and implementation for Zigzag Conversion problem.

Author: javadev

src/main/java/g0001_0100/s0006_zigzag_conversion/readme.md

@@ -36,4 +36,72 @@ string convert(string s, int numRows);
 
 *   `1 <= s.length <= 1000`
 *   `s` consists of English letters (lower-case and upper-case), `','` and `'.'`.
-*   `1 <= numRows <= 1000`
+*   `1 <= numRows <= 1000`
+
+To solve the Zigzag Conversion problem in Java using a `Solution` class, we'll follow these steps:
+
+1. Define a `Solution` class with a method named `convert`.
+2. Create an array of strings to represent each row of the zigzag pattern.
+3. Initialize variables to keep track of the current row (`row`) and the direction of traversal (`down`).
+4. Iterate through each character in the input string `s`.
+   - Append the current character to the string representing the current row.
+   - If we reach the first or last row, change the direction of traversal accordingly.
+   - Update the current row based on the direction of traversal.
+5. Concatenate the strings representing each row to form the final zigzag conversion.
+6. Return the concatenated string.
+7. Handle edge cases where the number of rows is 1 or the input string is empty.
+
+Here's the implementation:
+
+```java
+public class Solution {
+
+    public String convert(String s, int numRows) {
+        if (numRows == 1 || s.length() <= numRows) {
+            return s;
+        }
+
+        StringBuilder[] rows = new StringBuilder[numRows];
+        for (int i = 0; i < numRows; i++) {
+            rows[i] = new StringBuilder();
+        }
+
+        int row = 0;
+        boolean down = false;
+
+        for (char c : s.toCharArray()) {
+            rows[row].append(c);
+            if (row == 0 || row == numRows - 1) {
+                down = !down;
+            }
+            row += down ? 1 : -1;
+        }
+
+        StringBuilder result = new StringBuilder();
+        for (StringBuilder sb : rows) {
+            result.append(sb);
+        }
+
+        return result.toString();
+    }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+
+        // Test cases
+        String s1 = "PAYPALISHIRING";
+        int numRows1 = 3;
+        System.out.println("Example 1 Output: " + solution.convert(s1, numRows1));
+
+        String s2 = "PAYPALISHIRING";
+        int numRows2 = 4;
+        System.out.println("Example 2 Output: " + solution.convert(s2, numRows2));
+
+        String s3 = "A";
+        int numRows3 = 1;
+        System.out.println("Example 3 Output: " + solution.convert(s3, numRows3));
+    }
+}
+```
+
+This implementation provides a solution to the Zigzag Conversion problem in Java.