#include <iostream>
using namespace std;
void merge(int* a, int start, int mid, int end) {
int *tmp = new int[end-start+1];
int i = start;
int j = mid + 1;
int k = 0;
while(i <= mid && j <= end) {
if (a[i] <= a[j]) tmp[k++] = a[i++];
else tmp[k++] = a[j++];
}
while(i <= mid) tmp[k++] = a[i++];
while(j <= end) tmp[k++] = a[j++];
for (i = 0; i < k; i++)
a[start + i] = tmp[i];
delete[] tmp;
}
void mergeSortUp2Down(int* a, int start, int end) {
if(a==NULL || start >= end) return ;
int mid = (end + start)/2;
mergeSortUp2Down(a, start, mid);
mergeSortUp2Down(a, mid+1, end);
merge(a, start, mid, end);
}
int main() {
int i;
int a[] = {80,30,60,40,20,10,50,70};
int ilen = (sizeof(a)) / (sizeof(a[0]));
cout << "before sort:";
for (i=0; i<ilen; i++)
cout << a[i] << " ";
cout << endl;
mergeSortUp2Down(a, 0, ilen-1);
cout << "after sort:";
for (i=0; i<ilen; i++)
cout << a[i] << " ";
cout << endl;
return 0;
}
思路:归并排序采用经典的分治法,将一个数组不断的分解成n个小数组,通过处理小数组且合并,达到排序的目的。但是归并排序需要一个O(n)长度的辅助数组,也就是空间复杂度是O(n)。该算法的时间复杂度为O(nlogn),是一种稳定的排序算法。