LeetCode 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"

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O(n^2)级别的解法

使用遍历的方法，从左往右依次寻找，寻找的基本思想是先从一个字母开始，如果左边索引对应的数等于右边索引对应的数，则扩展左边索引和右边索引，左减1，右加1，直到左边或右边达到边界。并且记录相应的左边索引和子串的长度。

代码示例

public class Solution {
private int lo, maxLen;

public String longestPalindrome(String s) {
    int len = s.length();
    if (len < 2)
        return s;
    
    for (int i = 0; i < len-1; i++) {
        extendPalindrome(s, i, i);  //assume odd length, try to extend Palindrome as possible
        extendPalindrome(s, i, i+1); //assume even length.
    }
    return s.substring(lo, lo + maxLen);
}

private void extendPalindrome(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) {
        lo = j + 1;
        maxLen = k - j - 1;
    }
}}
        

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