32_最长有效括号JavaScript_最长有效括号字符串 javascript-优快云博客

本文链接：https://blog.youkuaiyun.com/sakumashirayuki/article/details/112635031

本文详细介绍了如何使用动态规划、栈以及正反遍历三种方法解决LeetCode上的最长有效括号问题，旨在帮助JavaScript开发者提升算法解决能力。

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方法一：动态规划

/**
 * @param {string} s
 * @return {number}
 */
var longestValidParentheses = function(s) {
    var len = s.length;
    if (len<=1)
        return 0;
    if (len===2)
    {
        if (s[0]=='('&&s[1]==')')
            return 2;
        else
            return 0;
    }
    //save the longest valid parenthese numbers at each position of the string
    var dp = new Array(len).fill(0);
    for(let i = 1; i < len; i++)
    {
        if(s[i]==')')
        {
            if(s[i-1]=='(')
            {
                if(i>=2)
                {
                    dp[i] = dp[i - 2] + 2;
                }else{
                    dp[i] = 2;
                }
            }else{ //s[i-1]=')'
                if((i - dp[i - 1] - 1)>=0 && s[i - dp[i - 1] - 1] == '(')
                {
                    if((i - dp[i - 1] - 2)>=0)
                        dp[i] = dp[i - 1] + dp[i - dp[i - 1] - 2] + 2;
                    else
                        dp[i] = dp[i - 1] + 2;
                }else{
                    dp[i] = 0;
                }
            }
        }
    }
    return Math.max(...dp);
};

方法二：栈

/**
 * @param {string} s
 * @return {number}
 */
var longestValidParentheses = function(s) {
    var len = s.length;
    if(len===0)
        return 0;
    var maxValidLength = 0;
    var stack = new Array(); // create empty array
    stack.push(-1);
    for(let i = 0; i < len; i++){
        if(s[i]=='('){
            stack.push(i);
        }
        else{ // right parenthesis
            var top = stack.pop();
            if((top==-1)||(s[top]==')'))
            {
                stack.push(i);
            }
            else // s[top]='('
            {
                var nextTop = stack.pop();
                let tempValidLength = i - nextTop;
                if (tempValidLength>maxValidLength)
                {
                    maxValidLength = tempValidLength;
                }
                stack.push(nextTop);
            }
        }
    }
    return maxValidLength;
};

方法三：正反遍历

/**
 * @param {string} s
 * @return {number}
 */
var longestValidParentheses = function(s) {
    var len = s.length;
    if(len===0)
        return 0;
    var left = 0, right = 0;
    var maxValidLength = 0;
    for(let i = 0; i < len; i++)
    {
        if(s[i]=='(')
            left++;
        else
            right++;
        if(left==right)
        {
            let tempValidLength = 2 * left;
            if(tempValidLength > maxValidLength)
                maxValidLength = tempValidLength;
        }
        else if(right > left)
        {
            left = 0;
            right = 0;
        }
    }
    left = 0;
    right = 0;
    for(let i = len - 1; i >= 0; i--){
        if(s[i]=='(')
            left++;
        else
            right++;
        if(left==right)
        {
            let tempValidLength = 2 * left;
            if(tempValidLength > maxValidLength)
                maxValidLength = tempValidLength;
        }
        else if(left > right)
        {
            left = 0;
            right = 0;
        }
    }
    return maxValidLength;
};