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leetCode(37):Implement Queue using Stacks

2024-09-18 10:22:01   218人阅读

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).
      两个栈，一个作为压入栈，一个作为弹出栈。当弹出栈为空时，把压入栈中的数据依次弹出并压入到弹出栈中。

假设两者均为空，说明队列为空。

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

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

    // Get the front element.
    int peek(void) {
        if (pop_stack.empty())
		{
			while (!push_stack.empty())
			{
				pop_stack.push(push_stack.top());
				push_stack.pop();
			}
			if (!pop_stack.empty())
				return pop_stack.top();
		}
		else
		{
			return pop_stack.top();
		}
		return 0;
    }

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


leetCode(37):Implement Queue using Stacks


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