ValidParentheses

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.

class Solution {
public:
    bool isValid(string s) {
        stack<char> sk;
        for(int i = 0; i < s.length(); i++) {
            if(s[i] == '(' || s[i] == '[' || s[i] == '{') {
                sk.push(s[i]);
            }
            else if(sk.empty() || abs(sk.top()-s[i]) > 2) {
                return false;
            }
            else {
                sk.pop();
            }
        }
        return sk.empty();
    }
};