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225. Implement Stack using Queues (Easy)

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.

 

Follow-up: Can you implement the stack using only one queue?

Companies: Google, Amazon, Bloomberg

Related Topics:
Stack, Design, Queue

Similar Questions:

• Implement Queue using Stacks (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/implement-stack-using-queues
// Author: github.com/lzl124631x
// Time: O(N) for push, O(1) for others
// Space: O(1)
class MyStack {
    queue<int> q;
public:
    MyStack() {}
    void push(int x) {
        q.push(x);
        for (int cnt = q.size() - 1; cnt > 0; --cnt) {
            q.push(q.front());
            q.pop();
        } 
    }
    int pop() {
        int ans = q.front();
        q.pop();
        return ans;
    }
    int top() {
        return q.front();
    }
    bool empty() {
        return q.empty();
    }
};

Solution 2.

// OJ: https://leetcode.com/problems/implement-stack-using-queues/
// Author: github.com/lzl124631x
// Time: O(N) for pop, O(1) for others
// Space: O(1)
class MyStack {
    queue<int> q;
    int topVal;
public:
    MyStack() {}
    void push(int x) {
        q.push(x);
        topVal = x;
    }
    int pop() {
        int ans = topVal;
        for (int i = 0, n = q.size(); i < n - 1; ++i) {
            topVal = q.front();
            q.push(topVal);
            q.pop();
        }
        q.pop();
        return ans;
    }
    int top() {
        return topVal;
    }
    bool empty() {
        return q.empty();
    }
};