class Solution { public: void push(int node) { stack1.push(node); } int pop() { while(!stack1.empty()) { stack2.push(stack1.top()); stack1.pop(); } if(!stack2.empty()) { int nRet = stack2.top(); stack2.pop(); while(!stack2.empty()) { stack1.push(stack2.top()); stack2.pop(); } return nRet; } return -1; } private: stack<int> stack1; stack<int> stack2; };