Problem

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.

Note

简单来说，建立堆栈stack。先用哈希表存对应的parentheses pair，再遍历字符串s，用switch语句判断。如果是前括号，放入堆栈stack；如果是后括号，那么查看stack顶端的元素和当前元素是否在哈希表对应，若不对应或堆栈为空，就返回false，否则弹出栈顶元素并继续循环。
循环结束后，若未抛出false，且堆栈为空，说明所有parenthese都已一一对应。

Solution

public class Solution {
    public boolean isValid(String s) {
        Map<Character, Character> map = new HashMap<>();
        map.put('(', ')');
        map.put('[', ']');
        map.put('{', '}');
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            Character ch = s.charAt(i);
            switch (ch) {
                case '{': case '[': case '(':
                    stack.push(ch);
                    break;
                case ')': case '}': case ']':
                    if (stack.isEmpty() || ch != map.get(stack.pop())) return false;
            }
        }
        return stack.isEmpty();
    }
}