输入两个已经按从小到大顺序排列好的字符串，编写一个合并两个字符串的函数，使合并后的字符串，仍然是从小到 大排列。

输入：两个已经排好顺序（升序）的两个字符串

输出：一个合并在一起的有序（升序）的字符串-Enter two small to large order of a good string, write a function to merge two strings, the string after the merger, is still small to large. Input: two have been in good order (ascending order) two strings output string: a combined order (ascending)