归并排序(Merge sort)

• 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:

  1. Divide—partition S into two sequences S1 and S2 of about n/2 elements each
  2. Recur—recursively sort S1 and S2
  3. Conquer—merge S1 and S2 into a sorted sequence