var alternate = function(list) {

    var index = 0;

    var list_size = list.length;

    var process = function(list_process) {

        // Search the next item different, remove and return this.

        var serchNextDifferent = function(number) {

            for (var i = index+1; i <= list_size; i++) {

                if (list_process[i] !== number) {

                    return list_process.splice(i,1);

                }

            }

        };

        // Search the next item different, remove and return this.

        var serchPrevDifferent = function(number, index) {

            for (var i = index-1; i >= 0; i--) {

                if (list_process[i] !== number && list_process[i] !== list_process[index] && number !== list_process[i-1] && i) {

                    return list_process.splice(i,1);

                }

            }

        };

        // Check if the current item and the prev are equals

        if (list_process[index-1] === list_process[index]) {

            var next = serchNextDifferent(list_process[index]);

            if (next.length) {

                list_process.splice(index, 0, next[0]);

            } else {

                var prev = serchPrevDifferent(list_process[index], index);

                if (prev && prev.length) {

                    list_process.splice(index-1, 0, prev[0]);

                } else {

                    list_process.push(list_process.splice(index, 1)[0]);    

                }

            }

        }

​

        // next

        if (list_size-1 !== index) {

            index++;

            return process(list_process);

        } else {

            return list_process; 

        }

    };

    return process(list);

};

​

console.log(alternate(['a','c','c','a','b','b']), ['a','c','a','b','c','b']);

console.log(alternate(['a','c','c','c','c','a']), ['a','c','a','c','c','c']);

​

console.log(alternate(['a','b','a','b','b','b','b','a']), ["a", "b", "a", "b", "a", "b", "b", "b"]);