4.1.1 Valid Parentheses

Notes:
	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: stack.
	*/

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