算法001

归并排序是分而治之的典型用法，其实现大致有4个步骤
1： 如果排序列表的长度为0或1，说明已经排序，直接返回。
2：把未排序列表从中间分为2个列表
3：递归的排序2个子列表
4：合并这2个子列表
归并排序的优点：
1：在小列表上排序所需的步骤元少于大列表排序的步骤
2：从2个有序的列表中构建有序列表所需步骤远少于未排序的2个的合并步骤

function mergesort(m)
var list left, right, result
if length(m) ≤ 1
return m

var middle = length(m) / 2
for each x in m up to middle
add x to left
for each x in m after middle
add x to right
left = mergesort(left)
right = mergesort(right)
result = merge(left, right)
return result


function merge(left,right)
var list result
while length(left) > 0 and length(right) > 0
if first(left) ≤ first(right)
append first(left) to result
left = rest(left)
else
append first(right) to result
right = rest(right)
end while
if length(left) > 0
append rest(left) to result
if length(right) > 0
append rest(right) to result
return result