图片.png

function f(str){
    var len = str.length
    var dp = new Array(len)
    //确定边界
    for(let i = 0; i< len; i++){
        var row = new Array(len).fill(0)
        row[i]= 1
        if(i < len-1){
            str[i+1] === str[i] ? row[ i+1 ] = 1: row[ i+1 ] = 0
        }
        dp[i] = row.slice()
    }
    //填写表格
    var tar = str[0] || null
    for(let i = 0; i< len; i++){

        for(let j = i+2; j < len; j++){
            if(str[i] === str[j]){
                dp[i][j] = dp[i+1][j-1]
                if(dp[i][j] === 1 && tar.length < (j-i+1)){
                    tar = str.substring(i, j+1)
                }
            }else {
                dp[i][j] = 0
            }
        }
    }
    return tar
}

var res = f('abccbd')
console.log(res)

图片.png