LeetCode - 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.

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public class Solution {

    public boolean isValid(String s) {

        if(s==null) {

            return true;

        }

        Stack<Character> stk = new Stack<Character>();

        for(int i=0;i<s.length();i++) {

            char c= s.charAt(i);

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

                stk.push(c);

            } else if(stk.isEmpty()) {

                return false;

            } else {

                char c2 = stk.pop();

                if(!((c==')'&&c2=='(') || (c=='}'&&c2=='{') || (c==']'&&c2=='['))) {

                    return false;

                }

            }

        }

        if(stk.isEmpty()) {

            return true;

        } else {

            return false;

        }

    }

}