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).
跟前面用队列实现堆原理是差不多的。
class MyQueue {
Stack<Integer> s1 = new Stack<Integer>();
Stack<Integer> s2 = new Stack<Integer>();
// 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() {
while (s1.size() > 1)
s2.push(s1.pop());
s1.pop();
while (s2.size() > 0)
s1.push(s2.pop());
}
// Get the front element.
public int peek() {
while (s1.size() > 0)
s2.push(s1.pop());
int n = s2.peek();
while (s2.size() > 0)
s1.push(s2.pop());
return n;
}
// Return whether the queue is empty.
public boolean empty() {
return s1.isEmpty();
}
}
