词频统计：输入任意长度的英文文本(空行表示结束)，输出出现频率最高的前三个单词(以空格分隔单词)及其出现次数，相同次序的按字母序输出。
提示：链表，动态内存分配，排序
输入1：
I am sinking in deep mud
and there is no solid ground
I am out in deep water
and the waves are about to drown me.
I am worn out from calling for help
and my throat is aching
I have strained my eyes
looking for your help
输出1：
I 4
am 3
and 3
提示：链表，动态内存分配，排序
输入1：
I am sinking in deep mud
and there is no solid ground
I am out in deep water
and the waves are about to drown me.
I am worn out from calling for help
and my throat is aching
I have strained my eyes
looking for your help
输出1：
I 4
am 3
and 3
