这里是 C# 版本的自下而上合并排序（更多细节可以参考我的博客@http://dream-e-r.blogspot.com/2014/07/mergesort-arrays-and-lists.html）
void BottomUpMergesort(int[] a)
{
int[] temp = new int[a.Length];
for (int runWidth = 1; runWidth < a.Length; runWidth = 2 * runWidth)
{
for (int eachRunStart = 0; eachRunStart < a.Length;
eachRunStart = eachRunStart + 2 * runWidth)
{
int start = eachRunStart;
int mid = eachRunStart + (runWidth - 1);
if(mid >= a.Length)
{
mid = a.Length - 1;
}
int end = eachRunStart + ((2 * runWidth) - 1);
if(end >= a.Length)
{
end = a.Length - 1;
}
this.Merge(a, start, mid, end, temp);
}
for (int i = 0; i < a.Length; i++)
{
a[i] = temp[i];
}
}
而合并定义为：
void Merge(int[] a, int start, int mid, int end, int[] temp)
{
int i = start, j = mid+1, k = start;
while((i<=mid) && (j<=end))
{
if(a[i] <= a[j])
{
temp[k] = a[i];
i++;
}
else
{
temp[k] = a[j];
j++;
}
k++;
}
while(i<=mid)
{
temp[k] = a[i];
i++;
k++;
}
while (j <= end)
{
temp[k] = a[j];
j++;
k++;
}
Assert.IsTrue(k == end+1);
Assert.IsTrue(i == mid+1);
Assert.IsTrue(j == end+1);
}
}
TopDownMergesort 仅供参考：
void TopDownMergesort(int[] a, int[] temp, int start, int end)
{
if(start==end)
{
//run size of '1'
return;
}
int mid = (start + end) / 2;
this.TopDownMergesort(a, temp, start, mid);
this.TopDownMergesort(a, temp, mid + 1, end);
this.Merge(a, start, mid, end, temp);
for(int i = start;i<=end;i++)
{
a[i] = temp[i];
}
}
单元测试
[TestMethod]
public void BottomUpMergesortTests()
{
int[] a = { 13, 4, 1, 3, 8, 11, 9, 10 };
this.BottomUpMergesort(a);
int[] b = { 1, 3, 4, 8, 9, 10, 11, 13 };
Assert.IsTrue(a.Length == b.Length);
for (int i = 0; i < a.Length; i++)
{
Assert.IsTrue(a[i] == b[i]);
}
List<int> l = new List<int>();
for (int i = 10; i >= 1; i--)
{
l.Add(i);
}
var la = l.ToArray();
this.BottomUpMergesort(la);
for (int i = 1; i <= 10; i++)
{
Assert.IsTrue(la[i - 1] == i);
}
l.Clear();
for (int i = 16; i >= 1; i--)
{
l.Add(i);
}
la = l.ToArray();
this.BottomUpMergesort(la);
for (int i = 1; i <= l.Count; i++)
{
Assert.IsTrue(la[i - 1] == i);
}
}