原题链接:https://leetcode-cn.com/problems/implement-queue-using-stacks/
分析和 225. 用队列实现栈 一样,只不过交换一下思考的角度而已
同样我们需要两个栈来实现队列
入队操作:此处一律默认将元素入栈到stackA中
出队操作:无非获取stackA的栈底元素1,则将所有元素从stackA出栈再入栈stackB,stackB最后出栈元素3即为出队操作
只不过我们还得需要将剩下的元素在返还给stackA,为什么?因为我们要保证元素的原先顺序
class MyQueue {
private Stack<Integer> stackA;
private Stack<Integer> stackB;
/** Initialize your data structure here. */
public MyQueue() {
stackA=new Stack<Integer>();
stackB=new Stack<Integer>();
}
/** Push element x to the back of queue. */
public void push(int x) {
stackA.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
while(!stackA.isEmpty()){
stackB.push(stackA.pop());
}
int ret=stackB.pop();
while(!stackB.isEmpty()){
stackA.push(stackB.pop());
}
return ret;
}
public int peek() {
while(!stackA.isEmpty()){
stackB.push(stackA.pop());
}
int ret=stackB.peek();
while(!stackB.isEmpty()){
stackA.push(stackB.pop());
}
return ret;
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stackA.isEmpty();
}
}