20. Valid Parentheses

题目：

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.

链接：http://leetcode.com/problems/valid-parentheses/

题解： 一道基本的使用stack的题目。 Time Complexity - O(n)， Space Complexity - O(n)

public class Solution {

    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 ++){

            int left = s.charAt(i);

            if(left == '(' || left == '[' || left == '{') 

                stack.push(s.charAt(i));

            else {

                if(stack.isEmpty())

                    return false;

                char right = stack.pop(); 

                if(left == ']' && right != '[')

                    return false;

                else if(left == '}' && right != '{')

                    return false;

                else if (left == ')' && right != '(')

                    return false;

            }

        }

        

        if(!stack.isEmpty())

            return false;

        return true;

    }

}