791. Custom Sort String——string

题目分析：S中先出现的T中先出现

class Solution(object):
    def repeatedNTimes(self, S, T):
        count = collections.Counter(T)
        answer = ""
        for s in S:
            answer += s*count[s]
            count[s] = 0
        for c in count:
            answer += c*count[c]
        return answer