LeetCode(23)-Implement Queue using Stacks（栈实现队列）

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).

思路：

代码：

class MyQueue {
Stack<Integer> s1 = new Stack<>();
Stack<Integer> s2 = new Stack<>();
// Push element x to the back of queue.
public void push(int x) {
s1.push(x);
}

// Removes the element from in front of queue.
public void pop() {
if(!s2.isEmpty()){
s2.pop();
}else{
if(s1.isEmpty()){
return;
}else{
while(!s1.isEmpty()){
int tmp = s1.pop();
s2.push(tmp);
}
s2.pop();
}
}

}

// Get the front element.
public int peek() {
if(!s2.isEmpty()){
return s2.peek();
}else{
if(s1.isEmpty()){
return -1;
}else{
while(!s1.isEmpty()){
int tmp = s1.pop();
s2.push(tmp);
}
return s2.peek();
}
}

}

// Return whether the queue is empty.
public boolean empty() {
return(s1.isEmpty() && s2.isEmpty());

}
}

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