add code of 5

Author: Li Li

top-interview-questions/5. Longest Palindromic Substring.cs

@@ -0,0 +1,27 @@
+/*
+for each point, try to expand to palindrom, and compare with maxLen.
+ */
+public class Solution {
+    private int start, maxLen;
+    public string LongestPalindrome(string s) {
+        int len = s.Length;
+        if (len < 2) return s;
+        for (int i = 0; i < s.Length - 1; i++) {
+            //assume odd length, try to extend Palindrome as possible
+            ExpandAroundCenter(s, i, i);
+            //assume even length.
+            ExpandAroundCenter(s, i, i + 1);
+        }
+        return s.Substring(start, maxLen);
+    }
+    private void ExpandAroundCenter(string s, int left, int right) {
+        while ( left >= 0 && right < s.Length && s[left] == s[right]) {
+            left--;
+            right++;
+        }
+        if (maxLen < right - left - 1) {
+            start = left + 1;
+            maxLen = right - left - 1;
+        }
+    }
+}