题目描述

Given a string containing just the characters ‘(‘, ‘)’, ‘{‘, ‘}’, ‘[‘ and ‘]’, determine if the input string is valid.

The brackets must close in the correct order, “()” and “()[]{}” are all valid but “(]” and “([)]” are not.

代码

    public static boolean isValid(String s) {

        // 字符串长度是奇数，肯定不合法，直接返回false
        if (s == null || s.length() % 2 == 1) {
            return false;
        }

        HashMap<Character, Character> map = new HashMap<Character, Character>();
        map.put('(', ')');
        map.put('[', ']');
        map.put('{', '}');

        Stack<Character> stack = new Stack<Character>();

        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);

            if (map.keySet().contains(c)) {
                stack.push(c);

            } else if (map.values().contains(c)) {

                if (!stack.empty() && map.get(stack.peek()) == c) {
                    stack.pop();
                } else {
                    return false;
                }
            }
        }

        return stack.empty();
    }