leetcode 232. Implement Queue using Stacks-栈模拟队列

原题链接：Implement Queue using Stacks
【思路】

用两个栈模拟一个队列：

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

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

// Get the front element.
public int peek() {
if(s2.isEmpty())
while (!s1.isEmpty()) s2.add(s1.pop());
return s2.peek();
}

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

17 / 17 test
cases passed. Runtime: 100
ms Your runtime beats 95.51% of javasubmissions.