题解 | #用两个栈实现队列#

class Solution
{
public:
    void push(int node) {
        if(stack1.empty()){
            help_push(node);
        } else {
            swap(stack1, stack2);
            help_push(node);
        }
    }

    int pop() {
        int data = stack2.top();
        stack2.pop();
        return data;
    }

private:
    stack<int> stack1;
    stack<int> stack2;
    
    void help_push(int node){
        while(!stack2.empty()){
            int data = stack2.top();
            stack1.push(data);
            stack2.pop();
        }
        stack2.push(node);
        while(!stack1.empty()){
            int data = stack1.top();
            stack2.push(data);
            stack1.pop();
        }
    }
};

有两种思路，一种是入栈完成后把出队顺序就维持好，到时候直接出队；另外一种是入栈随便，出栈的时候准备好出栈队列。第二种更简单。