Equal Number of 0's, 1's and 2's

The problem. Given a string s made only of the characters 0, 1 and 2, count the substrings that contain equally many 0s, 1s and 2s.

Example

s = "0102010"    ->  2    ("102" and "201")
s = "102100211"  ->  5
s = "0011"       ->  0    (no 2s at all)