在主函数中输入一个字符串main_str，调用函数将main_str中最长的单词取出放入sub_str中（要求被调函数参数为main_str和sub_str），在主函数中输出结果字符串sub_str。假定输入的字符串main_str中单词以一个或多个空格分开。若有多个单词长度相同，输出最后一个。

-Enter in the main function of a string main_str, calling function will main_str the longest word out into sub_str (the called function parameters required for the main_str and sub_str), the output string sub_str in the main function. Assume that the input string main_str in one or more words separated by spaces. If more than one word of the same length, the output of the last one.