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  • 5. Longest Palindromic Substring

    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"
    
     
    M1: dp
    dp[i][j]表示s.substring(i, j)是否是一个palindromic string
    time: O(n^2), space: O(n^2)
     
    M2: expand around corner
    遍历s,对每一个可能的位置都用helper function check能否expand around corner并找出expand的长度。因为center可能在字母位置也可能在两个字母中间位置,对两个位置都要expand并找出较大的。最后更新一下start, end的位置,如果len > start - end
    time: O(n^2), space: O(1)
    class Solution {
        public String longestPalindrome(String s) {
            if(s == null || s.length() < 1) {
                return "";
            }
            int start = 0, end = 0;
            for(int i = 0; i < s.length(); i++) {
                int len1 = helper(s, i, i);
                int len2 = helper(s, i, i + 1);
                int len = Math.max(len1, len2);
                if(len > end - start) {
                    start = i - (len - 1) / 2;
                    end = i + len / 2;
                }
            }
            return s.substring(start, end + 1);
        }
        
        private int helper(String s, int left, int right) {
            int l = left, r = right;
            while(l >= 0 && r < s.length() && s.charAt(l) == s.charAt(r)) {
                l--;
                r++;
            }
            return r - l - 1;
        }
    }
     
    M3: Manacher's Algorithm
    time: O(n), space: O()
     
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  • 原文地址:https://www.cnblogs.com/fatttcat/p/10317768.html
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