【Leetcode】20. Valid Parentheses【棧】

20. Valid Parentheses

• Total Accepted: 131984
• Total Submissions: 427751
• Difficulty: Easy

Given a string containing just the characters '(', ')','{', '}', '[' and ']', determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but"(]" and "([)]" are not.

思路：

遇到左括號入棧，遇到右括號時匹配棧頂元素出棧。

1、棧頂不匹配，直接return false；

2、最後棧裏面還有元素，return false；

3、入棧的時候需要注意，棧爲空時直接入棧，遇到右括號直接入棧。

代碼：

public class Solution {
    public boolean isValid(String s) {
        if(s == null) return true;
        Stack<Character> ss = new Stack<Character>();
        int len = s.length();
        for(int i = 0; i < len; i++){
            if(ss.empty()) ss.push(s.charAt(i));
            else if(s.charAt(i) == '('||s.charAt(i) == '{'||s.charAt(i) == '[') ss.push(s.charAt(i));
            
            else if(ss.peek() == '(' && s.charAt(i) == ')')  ss.pop();
            
            else if(ss.peek() == '[' && s.charAt(i) == ']')  ss.pop();
            
            else if(ss.peek() == '{' && s.charAt(i) == '}')  ss.pop(); 
            
            else{
                return false;
            }
        }
        if(ss.empty()) return true;
        else
        return false;
    }
}