二分查找的递归写法

常见的二分查找：

int binary_search(int *arr, int n, int x) {
    int head = 0, tail = n - 1, mid;
    while (head <= tail) {
        mid = (head + tail) / 2;
        if (arr[mid] == x) return mid;
        if (arr[mid] < x)
            head = mid + 1;
        else
            tail = mid - 1;
    }
    return -1;
}

改为递归：

int binary_search_recursion(int *arr, int l, int r, int x) {
    if (l > r) return -1;
    int mid = (l + r) / 2;
    if (arr[mid] == x) return mid;
    if (arr[mid] < x)
        l = mid + 1;
    else
        r = mid - 1;
    binary_search_recursion(arr, l, r, x);
}