LeetCode刷题笔录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.

用一个stack来记录左括号的index。遇到右括号后pop出栈顶得左括号。然后会有以下两种情况：

1.stack变为空。那么length = i - lastInvalidRight;

2.stack不为空。那么length = i - stack.peek();

上面两种情况分别对应的是")()()"以及"(()()"这样子。当然"((()))"也是可以处理的。

public class Solution {
public int longestValidParentheses(String s) {
if(s == null || s.length() == 0)
return 0;

char[] arr = s.toCharArray();
Stack<Integer> stack = new Stack<Integer>();
int max = 0;
int lastInvalidRight = -1;
for(int i = 0; i < arr.length; i++){
//push left paren's index onto stack
if(arr[i] == '('){
stack.push(i);
}
//right paren, pop the first left paren and calculate index difference
else{
if(stack.isEmpty()){
lastInvalidRight = i;
continue;
}
else{
stack.pop();
if(stack.isEmpty())
max = Math.max(max, i - lastInvalidRight);
else
max = Math.max(max, i - stack.peek());
}
}
}
return max;
}
}