记录一下

```C
#include<stdio.h>
int main() {
	int a[100], key, index = -1, high, low, mid, i, m;
	scanf("%d", &m);
	for (i = 0; i < m; i++) {
		scanf("%d", &a[i]);
	}
	scanf("%d", &key);
	low = 0; high = key-1;
	while (low <= high) {	//目的是防止数组中没有查找的元素，如果没有需要的元素最终low会大于high
		mid = (low + high) / 2;
		if (a[mid] == key) {
			index = mid; break;
		}
		else if (a[mid] <= key) {
			low = mid + 1;		//因为mid已经比较过所以+1
		}
		else if (a[mid] > key) {
			high = mid - 1;		//同上
		}
		else {
			index = -1; break;
		}
	}
	if (index == -1) {
		printf("NULL");
	}
	else
		printf("%d", index);

	return 0;
}

参考

懒猫老师-C语言-二分查找（折半查找）_哔哩哔哩_bilibili

C语言二分查找法 - 尼古拉斯宝莉 - 博客园 (cnblogs.com)