归并排序

思想: 分治

  1. 确定分界点mid=(l+r)/2
  2. 递归排序left,right
  3. 归并–合二为一

编程:

​ 对两个有序序列合并到第三个数组res里，双指针分别指向两个有序序列min1,min2，比较较小者插入到res数组里

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void merge_sort(int q[], int l, int r)
{
    if (l >= r) return;

    int mid = l + r >> 1;
    merge_sort(q, l, mid);
    merge_sort(q, mid + 1, r);

    int k = 0, i = l, j = mid + 1;
    while (i <= mid && j <= r)
        if (q[i] <= q[j]) tmp[k ++ ] = q[i ++ ];
        else tmp[k ++ ] = q[j ++ ];

    while (i <= mid) tmp[k ++ ] = q[i ++ ];
    while (j <= r) tmp[k ++ ] = q[j ++ ];

    for (i = l, j = 0; i <= r; i ++, j ++ ) q[i] = tmp[j];
}