递归分治

最典型的分治算法是归并排序

void mergeSort(vector<int>& nums, int left, int right)
{
    int mid = left + (right - left) / 2;
    // 1. 分：将数组分为左、右两个子数组，分别递归排序
    mergeSort(nums, left, mid);
    mergeSort(nums, mid + 1, right);
    // 2. 合：将排好序的左、右子数组进行合并
    merge(nums, left, mid, right);
}