Valid Parentheses

use stack (be careful of empty stack)

public class Solution {
    public boolean isValid(String s) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(s == null||s.length() == 0)
            return true;
            
        Stack<Character> mystack = new Stack<Character>();
        char[] mychar = s.toCharArray();
        for(int i = 0; i < mychar.length; i++)
        {
            if(mychar[i] == '('||mychar[i] == '{'||mychar[i] == '[')
                mystack.push(mychar[i]);
            else if(mychar[i] == ')')
            {
                if(mystack.isEmpty() || mystack.pop() != '(')
                    return false;
            }
            else if(mychar[i] == ']')
            {
                if(mystack.isEmpty() || mystack.pop() != '[')
                    return false;
            }
            else if(mychar[i] == '}')
            {
                if(mystack.isEmpty() || mystack.pop() != '{')
                    return false;
            }
            else
                return false;
        }
        if(mystack.isEmpty())
            return true;
        return false;
        
    }
}