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 d₁d₂… d_ke₁e₂… e_l, where d and e are the two digits and, d ≠ 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.