Time complexity：O(logn)
方法：二路归并(2-way merge)
对象：有序序列(sorted sequence), 即可以是有序向量(sorted vector)，也可以是列表(list)算法性质：迭代
步骤： 1.无序向量的递归分解(递归) 2.有序序列的逐层归并(迭代)
算法描述：（摘自Oxford一哥们写的DS教材～)
“Divide and conquer ” Merge sort has three steps to sort an input sequence S with n elements:
Divide—partition S into two sequences S1 and S2 of about n/2 elements each Recur—recursively sort S1 and S2Conquer—merge S1 and S2 into a sorted sequence
