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
        
          instk;    stack
         
           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
            
              que1;    queue
             
               que2;};