写一个函数，实现一个有序数组的二分查找。

#include <stdio.h>

int binary_search(int arr[],int key,int sz)
{
    int mid = 0;
    int left = 0;
    int right = sz - 1;
    while (left <= right)
    {
        int mid = left + (right - left) / 2;
        if (arr[mid] < key)
            left = arr[mid] + 1;
        else if (arr[mid] > key)
            right = arr[mid] - 1;
        else
            return mid;
    }
    return -1;
}
int main()
{
    int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    int sz = sizeof(arr) / sizeof(arr[0]);
    int key = 7;
    int ret = binary_search(arr, key, sz);
    if (ret == -1)
        printf("找不到!\n");
    else
        printf("找到了!下标为:%d\n", ret);
    return 0;
}