Leetcode20 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:

Open brackets must be closed by the same type of brackets.
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

思路

每一个]对应一个[,一个}对应一个{，一个)对应一个(，将对应关系放入map中。
遍历字符串中的字符，如果是{([,那么将该字符放入stack中，
如果不是，应当与栈顶元素进行匹配，通过map找到对应的字符，然后与栈顶元素进行匹配。

代码

class Solution {
    public boolean isValid(String s) {
        // if(s.length()==1) return false;
        Stack<Character> stack = new Stack<>();
        HashMap<Character,Character> map = new HashMap<>();
        map.put(')','(');
        map.put(']','[');
        map.put('}','{');
        for(int i=0;i<s.length();i++){
            if(s.charAt(i)=='('||s.charAt(i)=='{'||s.charAt(i)=='[')
                stack.add(s.charAt(i));
            else if(stack.empty()||map.get(s.charAt(i))!=stack.peek()){
                return false;
            }else{
                stack.pop();
                continue;
            } 
        }
        if(stack.empty())
            return true;
        return false;
    }
}