Algorithm 3 Queue Implemented using Two Stacks

Require: $S_1$ and $S_2$ are two stacks.

Ensure: Implementation of queue operations with $S_1$ and $S_2$ .

1:procedure Enqueue( $S_1, S_2, x$ )

2:Push( $S_2, x$ )

3:end procedure

5:procedure Dequeue( $S_1, S_2$ )

6:if StackEmpty( $S_1$ ) then

7:while not StackEmpty( $S_2$ ) do

8: $x =$ Pop( $S_2$ )

9:Push( $S_1, x$ )

10:end while

11:end if

12:return Pop( $S_1$ )

13:end procedure