Transfer from one stack to another

Updated Oct 7, 2026

Transfer pops from src and pushes onto dst until src is empty, or until a stop. Order reverses: dst's top is src's old bottom. Two transfers restore order. Queue-with-two-stacks uses that.

What to focus on

While src not empty: dst.push(src.pop()). Transferring from [] does nothing. Do not alias the stacks and pop/push the same one.

Worked examples

src = [4, 1, 9], dst = []

src = [4, 1, 9], to dst then back to src

src = [], dst = [7]

Try it

Write transfer(src, dst). src is the stack you pop. dst is the stack you push. Pop from src and push onto dst until src is empty. Print both stacks. A second transfer restores order. Transferring from [] is a no-op. src = [4, 1, 9], dst = [] → src [], dst [9, 1, 4]. src = [4, 1, 9], to dst then back to src → src [4, 1, 9]. src = [], dst = [7] → src [], dst [7].

src may be empty, which is a no-op. Values are integers. Transfer pops at most src.size times.