微软面试题 之 "Add back spaces"

有两个string，str1 = "i am a boy", str2 = "iamaboy"
写出下面函数的实现
char* AddBackSpaces(const char* str, const set<string>& words)
{
    ...
}
传入的str是"iamaboy"，words是["i", "am", "a", "boy"]，期望的输出是"i am a boy".