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[leetcode]Design-232. Implement Queue using Stacks

ack lis rip design supported sim ali standard mea

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) {  
        in.push(x);  
    }  
  
    // Removes the element from in front of queue.  
    void pop(void) {  
        if(out.empty())  
            trans();  
        out.pop();  
    }  
  
    // Get the front element.  
int peek(void) { if(out.empty()) trans(); return out.top(); } // Return whether the queue is empty. bool empty(void) { return out.empty() && in.empty(); } private: stack<int> in, out; void trans(){ while(!in.empty()){ out.push(in.top()); in.pop(); } } };

[leetcode]Design-232. Implement Queue using Stacks