20. Valid Parentheses

Problem:

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

Analysis:
Use stack, when meet left par, push right par in. When meet right par, we have to make sure the stack top is its left par. Otherwise the string is not valid.

Solution:
If the current c is right par, but the stack is empty, there is no left par in front of it. Thus the string is not valid.

class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length() == 0)
            return true;

        Stack<Character> stack = new Stack<>();
        for (char c: s.toCharArray()) {
            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();
    }
}