[LeetCode 32] Longest Valid Parentheses

題目

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

測試案例

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

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

思路

1. 采用棧數據結構。棧中存放各字符的下標。初始時裏面放入-1。

2. 從左至右依次遍歷每個字符。當前字符為左括號就進棧。當前字符為右括號時，如果棧中存在左括號，則出棧。否則，入棧。
3. 每當都元素，記下標為 i ，進棧時，就用 i - stack.peek() - 1 更新 max。

4. 遍歷結束後，需要用 n - stack.peek() - 1 更新 max。

代碼如下

class Solution {
    public int longestValidParentheses(String s) {        
        int max = 0, n = s.length(), temp, index = 0; 
        if(n == 0){
            return 0;
        }
        int[] stack = new int[n + 1];        
        stack[index++] = -1;
        for(int i = 0; i < n; i++){
            if(s.charAt(i) == ‘(‘ || (temp = stack[index - 1]) == -1 || 
               s.charAt(temp) == ‘)‘){                               
                if((temp = i - stack[index - 1] - 1) > max){
                    max = temp;
                }                
                stack[index++] = i;
            }
            else{
                index--;                
            }            
        }
        if((temp = n - stack[index - 1] - 1) > max){
            max = temp;
        }        
        return max;
    }
}
 

LeetCode 32 括號匹配