xyz => 1 sits on seat x, 2 on seat y, 3 on seat z
So the only possibilities are:
123: 1 sits on seat 1, 2 on seat 2, 3 on seat 3
213: 1 sits on seat 2, 2 on seat 1, 3 on seat 3
231: 1 sits on seat 2, 2 on seat 3, 3 on seat 1
321: 1 sits on seat 3, 2 on seat 2, 3 on seat 1
passenger 3 is happy twice out of 4, hence 50%
In the code, the happyLast variable counts the number of times the last passenger is happy ( happy == when he gets his assigned seat)
