Longest Valid Parentheses

1、题目

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.

2.算法

public int longestValidParentheses(String s){if (s == null || s.length() == 0){return 0;}int start = 0;int max = 0;LinkedList<Integer> stack = new LinkedList<Character>();for (int i = 0; i < s.length(); i++){if (s.charAt(i) == '('){stack.push(i);}else{if (stack.isEmpty()){start = i + 1;}else{stack.pop();max = stack.isEmpty() ? Math.max(max, i - start + 1) : Math.max(max, i - stack.peek());}}}return max;}

这道题的步骤是

1.遇到左括号进栈

2.如遇到右括号，如果此时栈为空，则不符合要求，此时最佳开始加一，如果栈不为空，则出栈，如果此时为空，计算此时最佳长度，如果不为空，则最佳长度为当前栈顶元素的位置下一位到当前元素的距离