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

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.

Input:"()(()"

Output:4

Expected:2

Input:"()(())"

Output:2

Expected:6

import java.util.Deque;
import java.util.LinkedList;
import java.util.Stack;

public class Solution {

public int longestValidParentheses(String s) {
// 用于记录待匹配的左括号和右括号的位置
Stack<Integer> st = new Stack<>();
int max = 0;
for (int i = 0; i < s.length(); i++) {

// 如是当前字符是右括号，并且记录栈非空，并且前一个字符是左括号
if (s.charAt(i) == ')' && !st.isEmpty() && s.charAt(st.peek()) == '(') {
// 左括号出栈
st.pop();
// 求最大值
max = Math.max(max, i - ((st.isEmpty()) ? -1 : st.peek()));
}
// 其它情况就将字符入栈
else {
st.push(i);
}
}
return max;
}
}