20. 有效的括号.md

class Solution {
    public boolean isValid(String s) {
        if (s.length() == 0) return true;
        Stack<Character> stack = new Stack<>();
        Map<Character, Character> map = new HashMap<>();
        map.put(')', '(');
        map.put(']', '[');
        map.put('}', '{');
        for (char c : s.toCharArray()) {
            if (c == '(' || c == '[' || c == '{')
                stack.push(c);
            else if (map.containsKey(c))
                if (stack.empty()) return false;
                else if (!(stack.pop() == map.get(c)))
                    return false;
        }
        return stack.empty();
    }
}