var isValid = function(s) {
    let map = {'(' : ')', '[': ']', '{':'}'};
    let stack = [];
    for(let ch of s){
        if(map[ch]){
		//pushing opening bracket first
            stack.push(map[ch]);
        }
        else if(stack.length > 0 && stack[stack.length-1] == ch){
		//if we have a matching closing bracket to the one we pushed earlier, we simply pop it.
            stack.pop();
        }
        else return false;
    }
	//if at last all values are poped i.e stack is empty, we have valid set of parentheses.
    return stack.length == 0;
};