LeetCode 232. Implement Queue using Stacks

[wupangyen]

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.

[wupangyen]

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

[wupangyen]

one input stack, push elements one out put stack for peek and pop

move element from input stack to output stack when needed

when we pop peek but the output stack is empty

more all the elements from input to output stack , reversing the order so it's correct for peek pop

O(1) amortized

[wupangyen]

class MyQueue { Stack inputStack; Stack outputStack;

/** Initialize your data structure here. */
public MyQueue() {
    inputStack = new Stack<Integer>();
    outputStack = new Stack<Integer>();

}

/** Push element x to the back of queue. */
public void push(int x) {
    inputStack.push(x);

}

/** Removes the element from in front of queue and returns that element. */
public int pop() {
    peek();
    return outputStack.pop();

}

/** Get the front element. */
public int peek() {

    if(outputStack.empty()){
            while(!inputStack.empty()){
                outputStack.push(inputStack.pop());
            }

    }

    return outputStack.peek();

}

/** Returns whether the queue is empty. */
public boolean empty() {
    return outputStack.empty() && inputStack.empty();
}

}

/**

• 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(); */