题目地址：

leetcode.com/problems/va…

Given a string s 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.

Example 1:

Input: s = "()"
Output: true

Example 2:

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

Example 3:

Input: s = "(]"
Output: false

Constraints:

• 1 <= s.length <= 104
• s consists of parentheses only '()[]{}'.

import java.util.Deque;
import java.util.LinkedList;

class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length() == 0) {
            return true;
        }

        Deque<Character> stack = new LinkedList<>();
		
        for (int i = 0; i < s.length(); i++) {
            char cur = s.charAt(i);
            // 如果当前字母是左括号，直接入栈
            if (cur == '(' || cur == '[' || cur == '{') {
                stack.push(cur);
            } else {
            	// 如果是右括号，若栈空直接返回false
                if (stack.isEmpty()) {
                    return false;
                }
				// 如果栈不空且与栈顶匹配，pop栈顶；否则返回false
                if (match(stack.peek(), cur)) {
                    stack.pop();
                } else {
                    return false;
                }
            }
        }
        
        return stack.isEmpty();
    }

    private boolean match(char c1, char c2) {
        if ((c1 == '(' && c2 == ')')
            || (c1 == '[' && c2 == ']'
            || (c1 == '{' && c2 == '}'))) {
                return true;
            }
        
        return false;
    }
}