20. Valid Parentheses

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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

//Time: O(n), Space: O(n)   
 public boolean isValid(String s) {
        if (s == null || s.length() == 0) {
            return true;
        }
        
        Stack<Character> stack = new Stack<Character>();
        
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            
            if (c == '(') {
                stack.push(')');
            } else if (c == '[') {
                stack.push(']');
            } else if (c == '{') {
                stack.push('}');
            } else {
                if (stack.isEmpty() || stack.pop() != c) {
                    return false;
                }
            }
        }
        
        return stack.isEmpty();//注意[的情况
    }