5. Longest Palindromic Substring

题目

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"

代码

   int st,end;
    @Test
    public void test5() {
        System.out.println(longestPalindrome2("babad"));
    }
    public String longestPalindrome2(String s) {
        int len = s.length();
        if(len < 1) {
            return s;
        }
        char[] chs = s.toCharArray();
        for (int i = 0; i < len; i++) {
            helper(chs,i,i);
            helper(chs,i,i + 1);
        }
        return s.substring(st,end+1);
    }

    private void helper(char[] chs,int l,int r) {
        while (l >= 0 && r < chs.length && chs[l] == chs[r]) {
            --l;
            ++r;
        }
        if(end - st < r - l  -2) {
            st = l + 1;
            end = r - 1;
        }
    }

思路是：中心扩展法
要么是一个数为中心，要么是两个数为中心，然后向两边扩展