232. Implement Queue using Stacks - Easy
阿新 • • 發佈：2019-01-04
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.
Example:
MyQueue queue = new MyQueue(); queue.push(1); queue.push(2); queue.peek(); // returns 1 queue.pop(); // returns 1 queue.empty(); // returns false
Notes:
- You must use only standard operations of a stack -- which means only
push to top,peek/pop from top,size, andis emptyoperations 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).
time: push() -- O(1), pop() -- amortized O(1), peek() -- amortized O(1), size() -- O(1)
space: O(n)
class MyQueue { LinkedList<Integer> s1; LinkedList<Integer> s2; int size; /** Initialize your data structure here. */ public MyQueue() { s1 = new LinkedList<>(); s2 = new LinkedList<>(); size = 0; } /** Push element x to the back of queue. */ public void push(int x) { s1.push(x); size++; } /** Removes the element from in front of queue and returns that element. */ public int pop() { if(s2.isEmpty()) { while(!s1.isEmpty()) { s2.push(s1.pop()); } } size--; return s2.pop(); } /** Get the front element. */ public int peek() { if(s2.isEmpty()) { while(!s1.isEmpty()) { s2.push(s1.pop()); } } return s2.peek(); } /** Returns whether the queue is empty. */ public boolean empty() { return size == 0; } } /** * 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(); */