[LeetCode] Longest Valid Parentheses

Description:
Given a string containing just the characters ‘(’ and ‘)’, find the length of the longest valid (well-formed) parentheses substring.

For “(()”, the longest valid parentheses substring is “()”, which has length = 2.

Another example is “)()())”, where the longest valid parentheses substring is “()()”, which has length = 4.

题目解析：
这里我们还是借助栈来求解，需要定义个start变量来记录合法括号串的起始位置，我们遍历字符串，如果遇到左括号，则将当前下标压入栈，如果遇到右括号，如果当前栈为空，则将下一个坐标位置记录到start，如果栈不为空，则将栈顶元素取出，此时若栈为空，则更新结果和i - start + 1中的较大值，否则更新结果和i - 栈顶元素中的较大值

code:

public int longestValidParentheses(String s) {        Stack<Integer> stack = new Stack<Integer>();        int max = 0;        int left = -1;        for(int j=0;j<s.length();j++){            if(s.charAt(j)=='(')stack.push(j);            else{                if(stack.isEmpty())left = j;                else{                    stack.pop();                     if(stack.isEmpty())max = Math.max(max, j-left);                     else max=Math.max(max,j-stack.peek());                }            }            return max;        }