add: LongestPalindromeInAStringExpanding

Author: Jer3myYu

java/Dynamic Programming/LongestPalindromeInAStringExpanding.java

@@ -0,0 +1,41 @@
+public class LongestPalindromeInAStringExpanding {
+    public String longestPalindromeInAStringExpanding(String s) {
+        int n = s.length();
+        int start, maxLen;
+        start = maxLen = 0;
+        for (int center = 0; center < n; center++) {
+            // Check for odd-length palindromes.
+            int oddStart, oddLength;
+            int[] oddResult = expandPalindrome(center, center, s);
+            oddStart = oddResult[0];
+            oddLength = oddResult[1];
+            if (oddLength > maxLen) {
+                start = oddStart;
+                maxLen = oddLength;
+            }
+            // Check for even-length palindromes.
+            if (center < n - 1 && s.charAt(center) == s.charAt(center + 1)) {
+                int evenStart, evenLength;
+                int[] evenResult = expandPalindrome(center, center + 1, s);
+                evenStart = evenResult[0];
+                evenLength = evenResult[1];
+                if (evenLength > maxLen) {
+                    start = evenStart;
+                    maxLen = evenLength;
+                }
+            }
+        }
+        return s.substring(start, start + maxLen);
+    }
+
+    // Expands outward from the center of a base case to identify the start 
+    // index and length of the longest palindrome that extends from this 
+    // base case.
+    private int[] expandPalindrome(int left, int right, String s) {
+        while (left > 0 && right < s.length() - 1 && s.charAt(left - 1) == s.charAt(right + 1)) {
+            left--;
+            right++;
+        }
+        return new int[]{left, right - left + 1};
+    }
+}