solve 5: longest palindromic substring

Author: andavid

vscode/5.longest-palindromic-substring.java

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+/*
+ * @lc app=leetcode id=5 lang=java
+ *
+ * [5] Longest Palindromic Substring
+ *
+ * https://leetcode.com/problems/longest-palindromic-substring/description/
+ *
+ * algorithms
+ * Medium (26.63%)
+ * Total Accepted:    505.9K
+ * Total Submissions: 1.9M
+ * Testcase Example:  '"babad"'
+ *
+ * Given a string s, find the longest palindromic substring in s. You may
+ * assume that the maximum length of s is 1000.
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input: "babad"
+ * Output: "bab"
+ * Note: "aba" is also a valid answer.
+ * 
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: "cbbd"
+ * Output: "bb"
+ * 
+ * 
+ */
+class Solution {
+  private int start = 0;
+  private int maxLen = 0;
+
+  public String longestPalindrome(String s) {
+    int len = s.length();
+    if (len < 2) {
+      return s;
+    }
+
+    for (int i = 0; i < len - 1; i++) {
+      extend(s, i, i);
+      extend(s, i, i+1);
+    }
+
+    return s.substring(start, start + maxLen);
+  }
+
+  public void extend(String s, int j, int k) {
+    while (j >= 0 && k < s.length() && s.charAt(j) == s.charAt(k)) {
+      j--;
+      k++;
+    }
+    if (maxLen < k - j - 1) {
+      maxLen = k - j - 1;
+      start = j + 1;
+    }
+  }
+}
+