一个简洁的归并排序实现模板

long long merge_sort(int l, int r) {
    int mid, p, q, i, j, len;

    if(l >= r)    return 0;
    mid = (l + r) >> 1;
    len = r - l + 1;

    p = l; q = mid + 1;
    j = l;


    for(i = 0; i < len; ++i) {
        if((q > r) || (num[p] < num[q] && p <= mid)) {
            t[j++] = num[p++];
        } else {
            t[j++] = num[q++];
        }
    }
    for(i = l; i <= r; ++i)    num[i] = t[i];

    return ans;
}