【leetcode-20】Valid Parentheses

题目描述

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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

题解如下：

思路：使用栈

代码如下

class Solution {
    private Map<Character,Character> mappings;
    
    public Solution(){
        this.mappings = new HashMap<>();
        this.mappings.put('}','{');
        this.mappings.put(']','[');
        this.mappings.put(')','(');
    }
    
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        
        for(int i=0;i<s.length();i++){
            char c = s.charAt(i);
            if(mappings.containsKey(c)){
                char topEle = stack.empty()? '#':stack.pop();
                if(topEle != mappings.get(c)){
                    return false;
                }
            } else {
                stack.push(c);
            }
        }
        
        return stack.isEmpty();
    }
}