思路: 用2 stack, 當stack1.push的時侯將stack的item放進stack2 中, 當stack1為空時放入item x (item x 在stack1 底部), 再將所有stack2 item放入stack1, --> 即重覆兩次filo --> fifo
class MyQueue {
Stack<Integer> stack1;
Stack<Integer> stack2;
/** Initialize your data structure here. */
//build a data structure
public MyQueue() {
this.stack1 = new Stack<>();
this.stack2 = new Stack<>();
}
/** Push element x to the back of queue. */
public void push(int x) {
while(!stack1.isEmpty()){
stack2.push(stack1.pop());
}
stack1.push(x);
while(!stack2.isEmpty()){
stack1.push(stack2.pop());
}
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
return stack1.pop();
}
/** Get the front element. */
public int peek() {
return stack1.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stack1.isEmpty();
}
}