How to implement 3 stacks with one array?

Question

Sometimes, I come across the following interview question: How to implement 3 stacks with one array ? Of course, any static allocation is not a solution.

Answer 1

I think you should divide array in 3 pieces, making head of first stack at 0, head of second stack at n/3, head of 3rd stack at n-1.

so implement push operation on :

1. first & second stack make i++ and for 3rd stack make i--;
2. If you encounter that first stack have no space to push, shift 2nd stack k/3 positions forward. Where k is the number of positions left to be filled in array.
3. If you encounter that second stack have no space to push, shift 2nd stack 2*k/3 positions backward. Where k is the number of positions left to be filled in array.
4. If you encounter that third stack have no space to push, shift 2nd stack 2*k/3 positions backward. Where k is the number of positions left to be filled in array.

We are shifting k/3 and 2*k/3 when no space is left so that after shifting of middle stack, each stack have equal space available for use.