LeetCode 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：找到最长的合法串的长度

思路：dp的思想，用d[i]表示从第i个位置开始匹配的长度，那么对于第i个来说，如果它是右括号的话，那么这个位置就是0，如果是做括号的话，那么就要跳过第i+1匹配的最长长度的位置来到j，看位置j是不是右括号，其次还要加上位置j+1的位置的匹配长度。

class Solution {
public:
    int longestValidParentheses(string s) {
        if (s.length() == 0) return 0;

        int ans = 0;
        int *d = new int[s.length()];
        for (int i = 0; i < s.length(); i++)
            d[i] = 0;
        d[s.length() - 1] = 0;
        for (int i = s.length() - 2; i >= 0; i--) {
            if (s[i] == ')')
                d[i] = 0;
            else {
                int j = i + 1 + d[i + 1];
                if (j < s.length() && s[j] == ')') {
                    d[i] = d[i+1] + 2;
                    if (j + 1 < s.length())
                        d[i] += d[j+1];
                }
            }
            ans = max(ans, d[i]);
        }

        return ans;
    }
};