32. Longest Valid Parentheses

Given a string containing just the characters '(' and ')', return the length of the longest valid (well-formed) parentheses substring.

 

Example 1:

Input: s = "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()".

Example 2:

Input: s = ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()".

Example 3:

Input: s = ""
Output: 0

 

如果是((()))，会输出6 

public int longestValidParentheses(String s) {
  int max = 0, start = 0;
  if(null == s)
    return 0;
  int len = s.length();
  Stack<Integer> stack = new Stack<>();
  for(int index = 0; index < len; index++) {
    //遇左括号(，压栈(栈中元素为当前位置所处的下标)
    if ('(' == s.charAt(index)) {
      stack.push(index);
      continue;
    }
    if (stack.isEmpty()) {
      start = index + 1;
      continue;
    } 
    stack.pop();
    if (stack.isEmpty()) {
      max = Math.max(max, index - start + 1);
    } else { // 如果是((())) 这种字符串，会走到此分支
      max = Math.max(max, index - stack.peek());
    }
  }
  return max;
}