https://github.com/emahtab/implement-queue-using-stacks
Implement Queue using Stacks
https://github.com/emahtab/implement-queue-using-stacks
leetcode problem-solving queue stack
Last synced: about 2 months ago
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Implement Queue using Stacks
- Host: GitHub
- URL: https://github.com/emahtab/implement-queue-using-stacks
- Owner: eMahtab
- Created: 2020-02-22T15:29:43.000Z (about 6 years ago)
- Default Branch: master
- Last Pushed: 2020-02-22T16:20:20.000Z (about 6 years ago)
- Last Synced: 2025-06-20T11:45:41.603Z (9 months ago)
- Topics: leetcode, problem-solving, queue, stack
- Size: 11.7 KB
- Stars: 1
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
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README
# Implement Queue using Stacks
## https://leetcode.com/problems/implement-queue-using-stacks
Implement the following operations of a queue using stacks.
1. push(x) -- Push element x to the back of queue.
2. pop() -- Removes the element from in front of queue.
3. peek() -- Get the front element.
4. empty() -- Return whether the queue is empty.
```
Example:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // returns 1
queue.pop(); // returns 1
queue.empty(); // returns false
```
**Notes:**
1. You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
2. Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
3. You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
## Implementation 1 : 2 Stacks
```java
class MyQueue {
/** Initialize your data structure here. */
private Stack stack = new Stack<>();
/** Push element x to the back of queue. */
public void push(int x) {
stack.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
Stack temp = new Stack<>();
while(!stack.isEmpty()){
temp.push(stack.pop());
}
int top = temp.pop();
while(!temp.isEmpty()){
stack.push(temp.pop());
}
return top;
}
/** Get the front element. */
public int peek() {
Stack temp = new Stack<>();
while(!stack.isEmpty()){
temp.push(stack.pop());
}
int top = temp.peek();
while(!temp.isEmpty()){
stack.push(temp.pop());
}
return top;
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stack.isEmpty();
}
}
```
## Implementation 1 : 2 Stacks (Improvements)
```java
class MyQueue {
/** Initialize your data structure here. */
private Stack stack = new Stack<>();
private Stack auxiliaryStack = new Stack<>();
private int front;
/** Push element x to the back of queue. */
public void push(int x) {
if(stack.isEmpty())
front = x;
stack.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
while(!stack.isEmpty()){
auxiliaryStack.push(stack.pop());
}
int top = auxiliaryStack.pop();
while(!auxiliaryStack.isEmpty()){
if(stack.isEmpty())
front = auxiliaryStack.peek();
stack.push(auxiliaryStack.pop());
}
return top;
}
/** Get the front element. */
public int peek() {
return front;
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stack.isEmpty();
}
}
```
## Implementation 2 : 2 Stacks (Much better approach)
```java
class MyQueue {
/** Initialize your data structure here. */
private Stack stack = new Stack<>();
private Stack auxiliaryStack = new Stack<>();
private int front;
/** Push element x to the back of queue. */
public void push(int x) {
if(stack.isEmpty())
front = x;
stack.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
if (auxiliaryStack.isEmpty()) {
while (!stack.isEmpty())
auxiliaryStack.push(stack.pop());
}
return auxiliaryStack.pop();
}
/** Get the front element. */
public int peek() {
if (!auxiliaryStack.isEmpty()) {
return auxiliaryStack.peek();
}
return front;
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stack.isEmpty() && auxiliaryStack.isEmpty();
}
}
```
# References :
1. https://www.youtube.com/watch?v=Wg8IiY1LbII
2. https://leetcode.com/articles/implement-queue-using-stacks