leetcode Question: Implement Queue using Stacks

Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

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, 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.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Analysis:

This kind of problem usually requires more than one data structure to implement the other data structure. In this problem, two stacks are enough to implement a queue.

The idea is keep push element into stack 1, then "pop" is called, put all the elements from stack 1 to stack 2. Then pop the top element in stack 2.  If "pop" is called again, not stack 2 is not empty, just pop the top element is enough. If stack 2 is empty, put all the elements in stack 1 to stack 2.  In short, stack 1 is used for "push", stack 2 is used for "pop" and "peek". We do the "move element from stack1 to stack 2" only when stack 2 is empty and "pop" or "peek" is called.


e.g., We call push(1), push(2), push(3), push(4), and push(5), stack 1 is filled all the five elements, then do pop(), since stack 2 is empty, move elements from stack 1 to stack 2, then pop the top element (now is 1) in stack2:

 Then we call push(6), push(7), push(8), push(9), and pop(), pop(), pop(), stack 1 is used for pushing, and stack 2 is used for popping :

Again, we call pop() (now 5 is popped out and no element in stack 2), and a peek() operation is called, stack 2 is empty, so push elements from stack 1 to stack 2, and return the top element as the peek:

Code(C++):

class Queue {

stack<int> st1;
stack<int> st2;

public:
    // Push element x to the back of queue.
    void push(int x) {
        st1.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if (!st2.empty()){
            st2.pop();
        }else{
            while (!st1.empty()){
                st2.push(st1.top());
                st1.pop();
            }
            st2.pop();
        }
    }

    // Get the front element.
    int peek(void) {
        if (!st2.empty()){
            return st2.top();
        }else{
            while (!st1.empty()){
                st2.push(st1.top());
                st1.pop();
            }
            return st2.top();
        }
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return (st1.empty() && st2.empty());
    }
};

Code(Python):

class Queue(object):
    def __init__(self):
        """
        initialize your data structure here.
        """
        self.st1 = []
        self.st2 = []
        

    def push(self, x):
        """
        :type x: int
        :rtype: nothing
        """
        self.st1.append(x)
        

    def pop(self):
        """
        :rtype: nothing
        """
        if len(self.st2) == 0:
            while len(self.st1) != 0:
                self.st2.append(self.st1.pop())
        self.st2.pop()
                

    def peek(self):
        """
        :rtype: int
        """    
        if len(self.st2) == 0:
            while len(self.st1) != 0:
                self.st2.append(self.st1.pop())
        return self.st2[-1]
        

    def empty(self):
        """
        :rtype: bool
        """
        return not self.st1 and not self.st2