// 经典栈的应用
// 左括号,直接入栈,右括号判断是否匹配,最后
// 判断栈是否为空,为空就表示合法,否则非法
// 注意题目中提到了空串也合法

class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length() == 0)
            return true;
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == '(' || c == '[' || c == '{')
                stack.push(c);
            else if (!stack.isEmpty() && isMatch(stack.peek(), c)) {
                stack.pop();
            } else {
                return false;
            }
        }

        return stack.isEmpty();
    }

    public boolean isMatch(char a, char b) {
        if (a == '(' && b == ')')
            return true;
        if (a == '[' && b == ']')
            return true;
        if (a == '{' && b == '}')
            return true;
        return false;
    }
}