Find next greater number with same set of digits

EXAMPLES : 
Input:  n = "218765"
Output: "251678"

Input:  n = "1234"
Output: "1243"

Input: n = "4321"
Output: "Not Possible"

Input: n = "534976"
Output: "536479"

Following are few observations about the next greater number.
1) If all digits sorted in descending order, then output is always “Not Possible”.
For example, 4321. 2) If all digits are sorted in ascending order, then we need to swap last two digits.
For example, 1234.
3) For other cases, we need to process the number from rightmost side (why? because we need to find the smallest of all greater numbers)

i) the first place where the left-digit is less-than the right-digit in last example 534976, its 4.
ii)  find the smallest digit larger than 4 to the right , its 6.
iii) place it to the left of 4
iv) sort the digits to the right of 6.

CODE

#include 
#include 
#include 
using namespace std;
 
void swap(char *a, char *b){
    char temp = *a;
    *a = *b;
    *b = temp;
}
 
void findNextNumber(char num[], int n)
{
    int i, j;
    for (i = n-1; i > 0; i--)
        if (num[i] > num[i-1])
           break;
    if (i==0)
    {
        cout << "Next number is not possible";
        return;
    }
    int x = num[i-1], smallest = i;
    for (j = i+1; j < n; j++) if (num[j] > x && num[j] < num[smallest])
            smallest = j;
 
    swap(&num[smallest], &num[i-1]);
    sort(num + i, num + n);
 
    cout << "Next number with same set of digits is " << num;
 
    return;
}
 
// Driver program to test above function
int main(){
    char digits[] = "534976";
    int len = strlen(digits);
    findNextNumber(digits, len);
    return 0;
}
Output:
Next number with same set of digits is 536479