Also learn Stack permutations. Like Finding correct/ Incorrect sequence of stack permutation.
For example,
For the input sequence 1 2 3 4 5, find Invalid sequence in stack permutation ????
1) 4 3 5 2 1
2) 2 3 5 4 1
3) 5 4 3 2 1
4) 3 4 1 5 2
Answer is .... 4