查找有序数组中某个数首次出现的位置

int find(int *a, int len, int key)
{
    if (a == NULL || len < 1)
        return - 1;
    int low = 0;
    int high = len - 1;
    while(low <= high)
    {
        int mid = (low + high)/2;
        if (a[mid] == key && a[mid - 1] != key)
        {
            return mid;
        }
        else if (a[mid] >= key)
        {
            high = mid - 1;
        }
        else
        {
            low = mid + 1;
        }
    }
}