Valid Parentheses

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.

用一个stack，如果是括号左边push，右边就和pop出来的比较是否为一对（检查stack是否已经为空）。最后检查遍历完后，stack是否为空。

public class Solution {
    //Time: O(n)
    public boolean isValid(String s) {
        if (s == null || s.length() == 0) {
            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++) {
            if (map.containsKey(s.charAt(i))) {
                stack.push(s.charAt(i));
            } else {
                if (stack.isEmpty() || map.get(stack.peek()) != s.charAt(i)) {
                    return false;
                }
                stack.pop();
            }
        }
        if (!stack.isEmpty()) {
            return false;
        }
        return true;
    }
}