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.

括號匹配，遍歷ｓｔｒｉｎｇ，碰到左括號把左括號入棧，碰到右括號出棧，判斷出棧的字符和右括號是否匹配。

public class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        for (int i = 0; i < s.length(); i++) {
            switch (s.charAt(i)) {
                case ‘(‘:
                case ‘{‘:
                case ‘[‘:
                    stack.push(s.charAt(i));
                    break;
                case ‘)‘:
                    if (stack.empty() || stack.pop() != ‘(‘) {
                        return false;
                    }
                    break;
                case ‘}‘:
                    if (stack.empty() || stack.pop() != ‘{‘) {
                        return false;
                    }
                    break;
                case ‘]‘:
                    if (stack.empty() || stack.pop() != ‘[‘) {
                        return false;
                    }
                    break;
            }
        }
        if (stack.empty()) {
            return true;
        } else {
            return false;
        }
    }
}

20. Valid Parentheses