[leetcode] 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).

Use stack.

Push – o(1)

pop – o(n)

peek – o(1)

empty – o(1)

class Queue {
public:
    stack<int> s;
    int frontElement;
    // Push element x to the back of queue.
    void push(int x) {
        if(s.empty()){
            frontElement = x;
        }
        s.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        stack<int> tmp;
        while(!s.empty()){
            tmp.push(s.top());
            s.pop();
        }
        tmp.pop();
        if(!tmp.empty()){
            frontElement = tmp.top();
        }
        while(!tmp.empty()){
            s.push(tmp.top());
            tmp.pop();
        }
    }

    // Get the front element.
    int peek(void) {
        return frontElement;
    }

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

 

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