最长回文子串

[Sunny Fu]

[Pcjmy]

题目链接：https://leetcode.cn/problems/longest-palindromic-substring/description
时间复杂度：O(n^2)

/**
 * @param {string} s
 * @return {number}
 */
var longestPalindrome = function(s) {
    let ans='';
    let n=s.length;
    let f=new Array(n).fill(0).map(() => new Array(n).fill(0));
    for(let i=0;i<n;i++) f[i][i]=1;
    for(let len=2;len<=n;len++)
    {
        for(let i=0;i+len-1<n;i++)
        {
            let j=i+len-1;
            if(s[i]===s[j]&&f[i+1][j-1]+2===j-i+1) {
                f[i][j]=f[i+1][j-1]+2;
            }
        }
    }
    let l=n,r=0;
    for(let i=0;i<n;i++)
    {
        for(let j=0;j<n;j++)
        {
            if(f[i][j]===j-i+1)
            {
                if(j-i>r-l)
                {
                    l=i,r=j;
                }
            }
        }
    }
    return s.substring(l,r+1);
};

[Li Xiangyue]

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {
    let max = ''

    for(let i = 0; i < s.length; i++){
        helper(i,i)
        helper(i,i + 1)
    }
    function helper(left, right){
        while(left >= 0 && right < s.length && s[left] === s[right]){
            left--
            right++
        }
        const maxStr = s.slice(left + 1, right - 1 + 1) //这里+1-1只是为了便于理解
        if(maxStr.length > max.length){
            max = maxStr
        }
    }
    return max
};