[LeetCode] Valid Parentheses

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.

Solution:O(n)

public class Solution {
    public boolean isValid(String s) {
        Stack<String> stack = new Stack<String>();
        for(int i=0; i<s.length();i++){
            String str = ""+s.charAt(i);
            if(str.equals("(") || str.equals("{")|| str.equals("["))
                stack.push(str);
            else{
                if(stack.isEmpty())
                    return false;
                    
                String popup = stack.pop();
                if(str.equals(")") &&  popup.equals("(") )
                    continue;
                if(str.equals("]") &&  popup.equals("[") )
                    continue;    
                if(str.equals("}") &&  popup.equals("{") )
                    continue;    
                return false;    
            }    
        }
        return stack.isEmpty();
    }
}

Key Concept: Use stack to help make decision