LeetCode: Implement Stack using Queues

LeetCode: Implement Stack using Queues

Implement the following operations of a stack using queues.

1
2
3
4
push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.

Notes:

You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
public class ImplementStackUsingQueues {
private Queue<Integer> q1 = new LinkedList<Integer>();
private Queue<Integer> q2 = new LinkedList<Integer>();

// Push element x onto stack.
public void push(int x) {
if (empty()) {
q1.add(x);
return;
}

if (q1.isEmpty()) {
q1.add(x);
while (!q2.isEmpty()) {
q1.add(q2.poll());
}
} else {
q2.add(x);
while (!q1.isEmpty()) {
q2.add(q1.poll());
}
}
}

// Removes the element on top of the stack.
public void pop() {
if (empty()) {
return;
}
if (q1.isEmpty()) {
q2.poll();
} else {
q1.poll();
}
}

// Get the top element.
public int top() {
int result;
if (q1.isEmpty()) {
result = q2.peek();
} else {
result = q1.peek();
}
return result;
}

// Return whether the stack is empty.
public boolean empty() {
return q1.isEmpty() && q2.isEmpty();
}
}