LeetCode - 32 -Longest Valid Parentheses.md

Longest Valid Parentheses

題目

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.

Java代碼

public class Solution {
    public int longestValidParentheses(String s) {
        Stack<Integer> stack = new Stack<Integer>();
        int max = 0;
        int left = -1;
        for (int i = 0; i < s.length(); i++){
            if (s.charAt(i) == '('){
                stack.push(i);
            } else {                
                if (stack.isEmpty())
                    //如果棧爲空，當前')'不合法，重置left
                    left = i;
                else {
                    stack.pop();
                    //當棧中存在'('，彈出
                    if (stack.isEmpty())
                        max = Math.max(max, i - left);
                    else
                        max = Math.max(max, i - stack.peek());
                }
            }
        }
        return max;
    }
}