归并排序

归并的思想：

归：就跟分树一样，每次对半分，直到分到1位置

并：从下往上合并,从传入的数组中间分开，左边/右边已经排序好了，此时都从最先结点开始一个一个比大小再合并

要新建一个临时数组

先排序到临时数组再复制回原数组

 #include<stdio.h>
#include<stdlib.h>
void mergearray(int a[], int first, int mid, int last, int temp[])  
{  
    int i = first;
    int j = mid + 1;  

    int k = 0;  
      
    while (i <= mid && j <= last)  
    {  
        if (a[i] <= a[j])  
            temp[k++] = a[i++];  
        else  
            temp[k++] = a[j++];  
    }  
      
    while (i <= mid)  
        temp[k++] = a[i++];  
      
    while (j <= last)  
        temp[k++] = a[j++];  
      
    for (i = 0; i < k; i++)  //还原回去,从传入first开始还原
        a[first + i] = temp[i];  
}  

void mergesort(int a[], int first, int last, int temp[]) //临时数组 
{  
    if (first < last)  
    {  
        int mid = (first + last) / 2;  
        mergesort(a, first, mid, temp);    //左边有序  
        mergesort(a, mid + 1, last, temp); //右边有序  
        mergearray(a, first, mid, last, temp); //再将二个有序数列合并  
    }  
}  
  
bool MergeSort(int a[], int n)  //排序的接口
{  
    int *p = new int[n];  
    if (p == NULL)//临时数组  
        return false;  
    mergesort(a, 0, n - 1, p);  
    delete[] p;  
    return true;  
}  

int main()
{
	int a[10] = {48,151,13,15,17,18,14,45,46,85};
	MergeSort(a,10);
	
	for(int i=0;i<10;i++)
	{
		printf("%d ",a[i]);
	}
}