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Implement Queue using Stacks




Total Accepted: 44005  Total Submissions: 129236  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).

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思路：两个栈实现一个队列


c++ code:

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        s2.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if(s1.empty()) {
            while(!s2.empty()){
            int t = s2.top();
            s2.pop();
            s1.push(t);
            }
        }
        s1.pop();
    }

    // Get the front element.
    int peek(void) {
        if(s1.empty()) {
            while(!s2.empty()){
                int t = s2.top();
                s2.pop();
                s1.push(t);
            }
        }
        return s1.top();
    }

    // Return whether the queue is empty.
    bool empty(void) {
        if(s1.empty() && s2.empty()) return true;
        else return false;
    }
private:
    stack<int> s1,s2;
};


本文标签： implementLeetCodeStacksQueue

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