LeetCode 20. Valid Parentheses

20. Valid Parentheses

Difficulty:: Easy

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

Solution

Language: Java

class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length() == 0) {
            return true;
        }
        Deque<Character> stack = new ArrayDeque<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            switch(c) {
                case '(':
                    stack.push(')');
                    break;
                case '[':
                    stack.push(']');
                    break;
                case '{':
                    stack.push('}');
                    break;
                default:
                    if (stack.isEmpty()) {
                        return false;
                    } else if (stack.peek() == c) {
                        stack.pop();
                    } else {
                        return false;
                    }
            }
        }
        return stack.isEmpty();
    }
}

也可以考虑用数组作为栈