【LeetCode】20. Valid Parentheses

Difficulty:easy

 More:【目录】LeetCode Java实现

Description

https://leetcode.com/problems/valid-parentheses/

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

Intuition

Using a stack.

 

Solution

    public boolean isValid(String s) {
        if(s==null)
            return false;
        Stack<Character> stk = new Stack<Character>();
        for(Character c : s.toCharArray()){
            if(c=='(')
                stk.push(')');
            else if(c=='[')
                stk.push(']');
            else if(c=='{')
                stk.push('}');
            else if(stk.isEmpty() || stk.pop()!=c)
                return false;
        }
        return stk.isEmpty();
    }

　　

Complexity

Time complexity : O(n)

Space complexity : O(n)

 

 More:【目录】LeetCode Java实现