LeetCode Implement Queue using Stacks
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原题链接在这里:https://leetcode.com/problems/implement-queue-using-stacks/
本题与Implement Stack using Queues相对应。
用Stack implement queue时,可以采用两个stack,add时就是一直像stk1中压栈。
poll()时把stk1的所有元素逐个压入另一个stack stk2中,现在stk2最上面的元素就是最先来的的,pop()之后再把剩下的压回到stk1中。
peek()与poll()类似,但这次不用pop(), 只需peek()出一个值返回。
Note: 1. 中间peek()时 用到了int res = stk2.peek();
所以生成stack时一定要加上类型<Integer>, if using Stack stk2 = new Stack(), it will throw error.
AC Java:
class MyQueue { // Push element x to the back of queue. private Stack<Integer> stk1 = new Stack<Integer>(); private Stack<Integer> stk2 = new Stack<Integer>(); public void push(int x) { stk1.push(x); } // Removes the element from in front of queue. public void pop() { while(!stk1.empty()){ stk2.push(stk1.pop()); } stk2.pop(); while(!stk2.empty()){ stk1.push(stk2.pop()); } } // Get the front element. public int peek() { while(!stk1.empty()){ stk2.push(stk1.pop()); } int res = stk2.peek(); while(!stk2.empty()){ stk1.push(stk2.pop()); } return res; } // Return whether the queue is empty. public boolean empty() { return stk1.empty(); }}
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