折半查找

int binary_search(const int arr[], int start, int end, int key) {
    int mid;
    while (start <= end) {
        mid = start + (end - start) / 2; //直接平均可能溢位，所以用此算法
        if (arr[mid] < key)
            start = mid + 1;
        else if (arr[mid] > key)
            end = mid - 1;
        else
            return mid; 
    }
    return -1;
}