leetcode232. 用栈实现队列
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使用栈实现队列的下列操作:
push(x) -- 将一个元素放入队列的尾部。
pop() -- 从队列首部移除元素。
peek() -- 返回队列首部的元素。
empty() -- 返回队列是否为空。
示例:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // 返回 1
queue.pop(); // 返回 1
queue.empty(); // 返回 false
push操作搞一个辅助栈就好了
python
class MyQueue: def __init__(self): """ Initialize your data structure here. """ self.stack=[] self.help_stack=[] def push(self, x: int) -> None: """ Push element x to the back of queue. """ while self.stack: self.help_stack.append(self.stack.pop()) self.help_stack.append(x) while self.help_stack: self.stack.append(self.help_stack.pop()) def pop(self) -> int: """ Removes the element from in front of queue and returns that element. """ return self.stack.pop(); def peek(self) -> int: """ Get the front element. """ return self.stack[-1] def empty(self) -> bool: """ Returns whether the queue is empty. """ return not bool(self.stack) # Your MyQueue object will be instantiated and called as such: # obj = MyQueue() # obj.push(x) # param_2 = obj.pop() # param_3 = obj.peek() # param_4 = obj.empty()
Java
class MyQueue { Stack<Integer>pushStack=new Stack<>(); Stack<Integer>popStack=new Stack<>(); /** Initialize your data structure here. */ public MyQueue() { } /** Push element x to the back of queue. */ public void push(int x) { while(!popStack.isEmpty()){ pushStack.push(popStack.pop()); } pushStack.push(x); } /** Removes the element from in front of queue and returns that element. */ public int pop() { while(!pushStack.isEmpty()){ popStack.push(pushStack.pop()); } return popStack.pop(); } /** Get the front element. */ public int peek() { while(!pushStack.isEmpty()){ popStack.push(pushStack.pop()); } return popStack.peek(); } /** Returns whether the queue is empty. */ public boolean empty() { return pushStack.isEmpty()&&popStack.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(); */
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