LeetCode 32. Longest Valid Parentheses（最长有效括号）

原题网址：https://leetcode.com/problems/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.

方法：用from来表示当前有效的开始点，即一旦出现根本无法匹配的情况，from将向前推进。用栈来保存左括号的坐标，每配对成功一个就出栈一个。

public class Solution {
    public int longestValidParentheses(String s) {
        int[] stack = new int[s.length()];
        int size = 0;
        int max = 0;
        int from = 0;
        for(int i=0; i

另一种实现：

public class Solution {
    public int longestValidParentheses(String s) {
        char[] sa = s.toCharArray();
        int[] pos = new int[sa.length+1];
        int max = 0;
        int size = 0;
        pos[size++] = -1;
        for(int i=0; i 1) {
                size --;
                max = Math.max(max, i-pos[size-1]);
            } else {
                pos[0] = i;
            }
        }
        return max;
    }
}

另一种实现：

public class Solution {
    public int longestValidParentheses(String s) {
        char[] sa = s.toCharArray();
        int[] stack = new int[sa.length];
        int size = 0;
        int from = 0;
        int max = 0;
        for(int i = 0; i < sa.length; i++) {
            if (sa[i] == '(') {
                stack[size++] = i;
            } else if (size == 0) {
                from = i + 1;
            } else if (size == 1) {
                max = Math.max(max, i - from + 1);
                size--;
            } else {
                size--;
                max = Math.max(max, i - stack[size - 1]);
            }
        }
        return max;
    }
}