LeetCode Challenge #232. Implement Queue using Stacks

Day 39 of 100 Days LeetCode Challenge

LeetCode Challenge #232. Implement Queue using Stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

void push(int x) Pushes element x to the back of the queue.
int pop() Removes the element from the front of the queue and returns it.
int peek() Returns the element at the front of the queue.
boolean empty() Returns true if the queue is empty, false otherwise.

Notes:

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.
Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.

Example 1:

Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Constraints:

1 <= x <= 9
At most 100 calls will be made to push, pop, peek, and empty.
All the calls to pop and peek are valid.

Video Solution

Java Solution

				
					class MyQueue {
     private Stack<Integer> main;
     private Stack<Integer> helper;

    public MyQueue() {
        main = new Stack<>();
        helper = new Stack<>();
    }
    
    public void push(int x) {
        // add all elements of main to helper -- main --> helper 

        while(main.size()>0){
            helper.push(main.pop());
        }
        
        // add x to main 

        main.push(x);

        // add all elemets from helper to main -- helper ---> main 

        while(helper.size()>0){
            main.push(helper.pop());
        }
    }
    
    public int pop() {
        return main.pop();
    }
    
    public int peek() {
        return main.peek();
    }
    
    public boolean empty() {
        return main.size()==0;
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

Day 39 of 100 Days LeetCode Challenge

LeetCode Challenge #232. Implement Queue using Stacks

Video Solution

Java Solution

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