【LeetCode】232 & 225

232 - 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).

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

    // Removes the element from in front of queue.
    void pop(void) {
        stack<int> temp;
        while(stk.size()!=1){
            int x=stk.top();
            stk.pop();
            temp.push(x);
        }
        stk.pop();
        while(!temp.empty()){
            int x=temp.top();
            temp.pop();
            stk.push(x);
        }
    }

    // Get the front element.
    int peek(void) {
        stack<int> temp;
        while(stk.size()!=1){
            int x=stk.top();
            stk.pop();
            temp.push(x);
        }
        int x=stk.top();
        while(!temp.empty()){
            int x=temp.top();
            temp.pop();
            stk.push(x);
        }
        return x;
    }

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

225 - Implement Stack using Queues

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) {
        q.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        queue<int> temp;
        while(q.size()!=1){
            int x=q.front();
            q.pop();
            temp.push(x);
        }
        q=temp;
    }

    // Get the top element.
    int top() {
        queue<int> temp;
        while(q.size()!=1){
            int x=q.front();
            q.pop();
            temp.push(x);
        }
        int x=q.front();
        q=temp;
        q.push(x);
        return x;
    }

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