/**
 * @param {string} s
 * @return {string}
 */
var removeDuplicateLetters = function(s) {
   var map = new Map(), stack = [];
   for(var i = 0, length = s.length; i < length; i  ) {
       var current = s[i];
       if (map.get(current)) {
           map.set(current, map.get(current)   1);
       } else {
           map.set(current, 1);
       }
   }

    for (var i = 0, length = s.length; i < length; i  ) {
        var current = s[i];
        map.set(current, map.get(current) - 1);
        if (stack.includes(current) == false) {
            var j = stack.length;
            while(j-- && stack[j] > current && map.get(stack[j])) {
                stack.pop();
            }
            stack.push(current);
        }
    }
    return stack.join('');
};