Leetcode232. Implement Queue using Stacks[栈实现队列]

一、题目描述

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)

二、考察点
栈和队列

三、CODE描述

栈和队列

import java.util.Stack;
class MyQueue {

// Push element x to the back of queue.
Stack<Integer> stack1 =new Stack<Integer>(); // push
Stack<Integer> stack2 =new Stack<Integer>(); // pop
public void push(int x) {
stack1.push(x);
}

// Removes the element from in front of queue.
public void pop() {
if(stack2.empty()){
while(!stack1.empty()){
int top = stack1.peek();
stack1.pop();
stack2.push(top);
}
}
if(!stack2.empty()){
stack2.pop();
}
}

// Get the front element.
public int peek() {
if(stack2.empty()){
while(!stack1.empty()){
int top = stack1.peek();
stack1.pop();
stack2.push(top);
}
}
if(stack2.empty()) return -1;
else
return stack2.peek();

}

// Return whether the queue is empty.
public boolean empty() {

return stack1.empty()&&stack2.empty();
}
}

四、结果



越努力，越幸运。