20. Valid Parentheses

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.

public boolean isValid(String s) {
    if(s==null||s.length()==1)
    return false;
        Stack<Character> stack = new Stack<Character>();
        for(int i=0;i<s.length();i++){
        if(s.charAt(i)=='('||s.charAt(i)=='{'||s.charAt(i)=='['){
        stack.push(s.charAt(i));
        }else{
        if(stack.empty()){
        return false;
        }else{
        char c = s.charAt(i);
if((c==')'&&stack.pop()=='(')||(c==']'&&stack.pop()=='[')||(c=='}'&&stack.pop()=='{')){
        continue;
        }else{
        return false;
        }
        }        
        }
        }
        return stack.isEmpty();
    }