LeetCode Challenge #225. Implement Stack using Queues

Day 38 of 100 Days LeetCode Challenge

LeetCode Challenge #225. Implement Stack using Queues

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

void push(int x) Pushes element x to the top of the stack.
int pop() Removes the element on the top of the stack and returns it.
int top() Returns the element on the top of the stack.
boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue’s standard operations.

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

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

Video Solution

Java Solution

				
					class MyStack {
    private Queue<Integer> main;
    private Queue<Integer> helper;

    public MyStack() {
        main = new LinkedList<>();
        helper = new LinkedList<>();
    }
    
    public void push(int x) {
    //   moving all elements from main to helper 
        while(main.size()>0){
          helper.add(main.remove());
        }
    //   add x in main queue 
    
        main.add(x);
    //   moving all elements from helper to main

        while(helper.size()>0){
            main.add(helper.remove());
        }  
    }
    
    public int pop() {
        return main.remove();
    }
    
    public int top() {
        return main.peek();
    }
    
    public boolean empty() {
        if(main.size()==0){
            return true ;
        }else{
            return false;
        }
    }
}

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

Day 38 of 100 Days LeetCode Challenge

LeetCode Challenge #225. Implement Stack using Queues

Video Solution

Java Solution

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