二路归并排序

//二路归并排序，时间复杂度O(nlog2n),空间复杂度O(n),稳定，与序列初始状态无关。
#include<iostream>
using namespace std;
void merge(int a[], int low, int mid, int high){//low...mid    mid+1...high是两个有序序列，merge将两个有序序列合并成一个有序序列
    int b[100]; int i, j, k;
    for (int k = low; k <= high; k++)
        b[k] = a[k];
    for (i = low, j = mid + 1, k = i; i <= mid&&j <= high;){
        if (b[i] < b[j])
            a[k] = b[i++];
        else
            a[k] = b[j++];
        k++;
    }
    while (i <= mid) a[k++] = b[i++];
    while (j <= high) a[k++] = b[j++];
}
void mergeSort(int a[], int low, int high){
    if (low >= high) return;
    int mid = (low + high) / 2;
    mergeSort(a, low, mid);
    mergeSort(a, mid + 1, high);
    merge(a, low, mid, high);//将low到mid,mid到high合并
}
int main(){
    int a[] = { 9, 8, 7, 6, 5, 4, 3, 2, 1 };
    mergeSort(a, 0, 8);
    for (int i = 0; i <= 8; i++)
        cout << a[i] << endl;
}