题目：
Implement the following operations of a queue using stacks.
Notes:
push to top, peek/pop from top, size, and is empty operations are valid.
题意及分析：使用stack实现queue的功能。这里对于queue的push和enpty操作来说，和stack一样就行。对于pop和peek，由于stack是pop和peek都是从队尾来讲，然而queue是要的这两个操作时对于队首的元素来讲。所以这里需要使用另一个stack来讲当前stack倒序添加进新stack,对于最后一个元素 取值或者去掉，然后在将新stack倒序回去即可。具体看代码：
代码：
public class MyQueue {
Stack<Integer> stack;
/** Initialize your data structure here. */
public MyQueue() {
stack = new Stack<>();
}
/** Push element x to the back of queue. */
public void push(int x) {
stack.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
Stack<Integer> tempStack=new Stack<>();
int lenght=stack.size();
for(int i=0;i<lenght-1;i++){
tempStack.push(stack.peek());
stack.pop();
}
int res=stack.peek();
stack.pop();
int tempSize=tempStack.size();
for(int i=0;i<tempSize;i++){
stack.push(tempStack.peek());
tempStack.pop();
}
return res;
}
/** Get the front element. */
public int peek() {
Stack<Integer> tempStack=new Stack<>();
int lenght=stack.size();
for(int i=0;i<lenght-1;i++){
tempStack.push(stack.peek());
stack.pop();
}
int res=stack.peek();
stack.pop();
tempStack.push(res);
int tempSize=tempStack.size();
for(int i=0;i<tempSize;i++){
stack.push(tempStack.peek());
tempStack.pop();
}
return res;
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stack.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();
*/
[LeetCode] 232. Implement Queue using Stacks Java
原文：http://www.cnblogs.com/271934Liao/p/7029287.html