232. Implement Queue using Stacks,225. Implement Stack using Queues

232. Implement Queue using Stacks

Total Accepted: 27024 Total Submissions: 79793 Difficulty: Easy

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).

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

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

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

    // Return whether the queue is empty.
    bool empty(void) {
        return instk.empty() && outstk.empty();
    }
private:
    stack<int> instk;
    stack<int> outstk;
};

 

225. Implement Stack using Queues

Total Accepted: 25628 Total Submissions: 84579 Difficulty: Easy

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        if(que1.empty()){
            que2.push(x);
        }else{
            que1.push(x);
        }
    }

    // Removes the element on top of the stack.
    void pop() {
        if(que1.empty()){
            while(que2.size() != 1){
                que1.push(que2.front());
                que2.pop();
            }
            que2.pop();
        }else{
            while(que1.size() != 1){
                que2.push(que1.front());
                que1.pop();
            }
            que1.pop();
        }
    }

    // Get the top element.
    int top() {
        int x = 0;
        if(que1.empty()){
            while(!que2.empty()){
                x = que2.front();
                que2.pop();
                que1.push(x);
            }
        }else{
            while(!que1.empty()){
                x = que1.front();
                que1.pop();
                que2.push(x);
            }
        }
        return x;
    }

    // Return whether the stack is empty.
    bool empty() {
        return que1.empty() && que2.empty();
    }
private:
    queue<int> que1;
    queue<int> que2;
};