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(); } }LeetCode官方链接
