题目
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).
答案
class MyQueue {
Stack<Integer> stk1, stk2;
/** Initialize your data structure here. */
public MyQueue() {
stk1 = new Stack<Integer>();
stk2 = new Stack<Integer>();
}
/** Push element x to the back of queue. */
public void push(int x) {
stk1.push(x);
}
private void moveStack() {
// otherwise use values in first stack
while(!stk1.isEmpty()) {
stk2.push(stk1.pop());
}
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
if(!stk2.isEmpty()) {
return stk2.pop();
}
moveStack();
return stk2.pop();
}
/** Get the front element. */
public int peek() {
if(!stk2.isEmpty()) return stk2.peek();
moveStack();
return stk2.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stk1.isEmpty() && stk2.isEmpty();
}
}
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue obj = new MyQueue();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.peek();
* boolean param_4 = obj.empty();
*/
