[Java] LeetCode32 Longest Valid Parentheses

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.

题意：给定一串包含括号的字符串，求字长有效的括号串。能顺序匹配的。这一题刚开始想好久~~，用stack来做题，，很容易求出字符串中所有有效的括号长度。但是如何求有效子串呢？如果我们能找出无效括号的index，将有效的index减去上一个无效的index，那么就是该字符的有效子串。弄清楚这一点求最大的有效字符串也会变得很容易。

public int longestValidParentheses(String s) {if(s==null)return 0;int len=s.length();int i=0;Stack<Integer> stack=new Stack<Integer>();char ch;int res=0;while(i<len){ch=s.charAt(i);if(ch=='(')stack.push(i);//我们变换思路，将括号的index入栈else{if(!stack.isEmpty()&&s.charAt(stack.peek())=='(')//如果是’)'，且与stack顶括号匹配时，弹出{stack.pop();res=Math.max(stack.isEmpty()?i+1:i-stack.peek(),res);//为空的话，证明前面没有无效括号，将i+1;不为空的，前面有无效字符，减去无效字符的index}else{stack.push(i);}}i++;}return res;}

慢慢学会了长大。流转的时光，