[LeetCode]32 Longest Valid Parentheses

public class Solution {
public int longestValidParentheses(String s) {

if (s == null || s.isEmpty())
return 0;   // Invalid input

Stack<Integer> stack = new Stack<>();

int max = 0;
int last = -1;

char[] chars = s.toCharArray();
int len = chars.length;
for (int i = 0 ; i < len ; i ++)
{
if (chars[i] == '(')
{
stack.push(i);
}
else // ')', assuming no other char
{
if (stack.empty())
{
last = i;
continue;   // invalid
}

stack.pop();
if (stack.isEmpty())
{
// 如果stack是空，那么从上一个invalid位置到i都valid
// 即 i - (last + 1) + 1
max = Math.max(max, i - (last + 1) + 1);
}
else
{
// 如果stack 不为空， 那么从上一个 '(' 之后到i都valid
// 即 i - (stack.peek() + 1) + 1
max = Math.max(max, i - (stack.peek() + 1) + 1);
}
}
}

return max;
}
}