LeetCode 20. 有效的括号

题目地址：​​https://leetcode-cn.com/problems/valid-parentheses/​​

思路：匹配到则弹出栈顶，元素也不放，否则把元素放到栈里

AC代码：

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char a = s.charAt(i);
            if (stack.size() == 0) {
                stack.push(a);
                continue;
            }
            char peekChar = stack.peek();
            if ((peekChar == '(' && a == ')') || (peekChar == '[' && a == ']') ||
                    (peekChar == '{' && a == '}')) {
                stack.pop();
            } else {
                stack.push(a);
            }
        }
        return stack.size() == 0 ? true : false;
    }
}