Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 3 | Stacks and Queues
3.6 Write a program to sort a stack in ascending order. You should not make any assumptions
about how the stack is implemented. The following are the only functions that
should be used to write this program: push | pop | peek | isEmpty.
SOLUTION
pg 52
Sorting can be performed with one more stack. The idea is to pull an item from the original
stack and push it on the other stack. If pushing this item would violate the sort order of the
new stack, we need to remove enough items from it so that it’s possible to push the new
item. Since the items we removed are on the original stack, we’re back where we started. The
algorithm is O(N^2) and appears below.
1 public static Stack sort(Stack s) {
2 Stack r = new Stack();
3 while(!s.isEmpty()) {
4 int tmp = s.pop();
5 while(!r.isEmpty() && r.peek() > tmp) {
6 s.push(r.pop());
7 }
8 r.push(tmp);
9 }
10 return r;
11 }
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Cracking the Coding Interview | Data Structures