Sandwich numbers V37339

We define a sandwich number as a natural number n with only two different digits d and e, forming a sequence de₁e₂⋯ e_kd. That is, the digit d is the first and last digit of n (it’s the bread), and the digit e is repeated k ≥ 1 times in between the two d digits (it’s the filling). For example, 121 is a sandwich number with d = 1, e = 2, and k = 1. And 4004 is a sandwich number with d = 4, e = 0 and k = 2.

More examples of sandwich numbers: 7227, 41114, 966669, 10001, and 535.
Examples of numbers that are not sandwich numbers: 9, 12, 113311, 7878, 1234, 9991, 1000.

Implement a function is_sandwich that receives a natural number and returns true if it is a sandwich number and false otherwise.

The function header should be:

/**
 * @pre  n >= 0
 * @post returns true if n is a sandwich number, false otherwise
 */
bool is_sandwich(int n);

Observation

You only need to submit the requested function; the main program will be ignored.