You are given a string: candies, which is composed of chars from the English alphabet. If you see two consecutive candies that are the same, you pop them from left to right, until you cannot pop any more candies. Return the resulting string.
// candies = "abba"
// "abba" - pop the two b's -> "aa"
// "aa" - pop the two a's -> ""
// return ""
// candies = "abcd"
// return "abcd"