用c语言写从键盘输入一整数，然后在一个整型一维数组a[20]中，用折半查找法找出该数是数 组中第几个元素的值。如果该数不在数组中，则打印”Nofound”。

#include <stdio.h>

int binary_search(int a[], int len, int target) {
    int left = 0, right = len - 1;
    while (left <= right) {
        int mid = left + (right - left) / 2;
        if (a[mid] == target) {
            return mid;
        } else if (a[mid] < target) {
            left = mid + 1;
        } else {
            right = mid - 1;
        }
    }
    return -1;
}

int main() {
    int a[20];
    int n, target;
    printf("请输入20个整数：\n");
    for (int i = 0; i < 20; i++) {
        scanf("%d", &a[i]);
    }
    printf("请输入要查找的整数：");
    scanf("%d", &target);
    n = binary_search(a, 20, target);
    if (n == -1) {
        printf("Nofound\n");
    } else {
        printf("%d\n", n+1);
    }
    return 0;
}