Bicolor numbers T80784

We define a bicolor number as a natural number $n$ with only two different digits that are repeated in two blocks (or "colors"). More formally, the sequence of digits of $n$ is $d_1d_2\ldots d_ke_1e_2\ldots e_l$, where $d$ and $e$ are the two digits and, $d \neq e$, $k > 0$ and $l > 0$.

Examples of bicolor numbers: 7722, 44111, 666699, 277, and 45.
Examples of number which are not bicolor: 121, 113311, 7878, 1234, 7, 99910.

Implement a function is_bicolor that receives a natural number and determines if it is bicolor. The function receives a number $n > 0$ and returns true if it is bicolor and false otherwise.

The function header must be exactly:

/**
* @pre n >= 0
* @post returns true if n is bicolor, false otherwise
*/
bool is_bicolor(int n);

Observation

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