JS Likou classic 100 questions - the longest palindrome substring

 Given a string s, find sthe longest palindromic substring in .

Example 1:

Input: s = "babad"
Output: "bab"
Explanation: "aba" is also the answer to the question.

Example 2:

Input: s = "cbbd"
Output: "bb"

hint:

    1 <= s.length <= 1000
    s consists of numbers and letters only

This question really can't be done without looking at the idea. . .

1. understand the meaning of the question
   • Title Given a string
   • We need to find the longest palindrome in this string
   • Palindrome: A string that reads the same forwards and backwards
2. the whole idea
   • I solve the problem based on a core idea of ​​the palindrome: spread from the center point to both sides to find the palindrome string. This direction is equivalent to enumerating every point as the center point.
   • If the palindrome string is odd, such as "bab", its center point has only one "a", so it spreads from "a" to both sides
   • If the palindrome string is even, such as "baab", its center point has two "aa", so it spreads from "aa" to both sides
   • Write an auxiliary function to find the palindrome string, call the auxiliary function when the center point is determined, and directly return the found palindrome string
   • Compare the palindrome string found each time with the previous one, and keep whoever is longer

/**
 * @param {string} s
 * @return {string}
 */
var longestPalindrome = function(s) {
    let str = ""
    for(let i=0; i<s.length; i++){//abba
        //穷举中心点
        let subString1 = findHuiWen(s,i,i)
        let subString2 = findHuiWen(s,i,i+1)
        str = str.length > subString1.length ? str : subString1
        str = str.length > subString2.length ? str : subString2
    }
    return str
};
function findHuiWen(s, left, right){
    while(left >=0 && right < s.length) {
        if(s.charAt(left) == s.charAt(right)) {
            left--
            right++
        } else {
            break
        }
    }//执行结束left和right都回退一个，但是substring方法左闭右开[,)，因此right保持不变
    return s.substring(left+1,right)
}

Knowledge points used:

String.prototype.charAt()

 The charAt() method returns the specified character from a string.

str.charAt(index)

String.prototype.substring()

substring()method returns a subset of a string between the start index and the end index, or a subset from the start index until the end of the string.

str.substring(indexStart[, indexEnd])