[LeetCode] 232. Implement Queue using Stacks 用栈来实现队列

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).

用栈来实现队列，对比类似题目225. Implement Stack using Queues 是反过来用。

Java:

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class MyQueue {        Stack<Integer> temp = new Stack<Integer>();     Stack<Integer> value = new Stack<Integer>();        // Push element x to the back of queue.     public void push(int x) {         if(value.isEmpty()){             value.push(x);         }else{             while(!value.isEmpty()){                 temp.push(value.pop());             }                value.push(x);                while(!temp.isEmpty()){                 value.push(temp.pop());             }            }     }        // Removes the element from in front of queue.     public void pop() {         value.pop();     }        // Get the front element.     public int peek() {         return value.peek();     }        // Return whether the queue is empty.     public boolean empty() {         return value.isEmpty();     } }

 Python:

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class Queue:     # initialize your data structure here.     def __init__(self):         self.A, self.B = [], []       # @param x, an integer     # @return nothing     def push(self, x):         self.A.append(x)       # @return an integer     def pop(self):         self.peek()         return self.B.pop()               # @return an integer     def peek(self):         if not self.B:             while self.A:                 self.B.append(self.A.pop())         return self.B[-1]               # @return an boolean     def empty(self):         return not self.A and not self.B

C++:

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class Queue { public:     // Push element x to the back of queue.     void push(int x) {         stack<int> tmp;         while (!s.empty()) {             tmp.push(s.top());             s.pop();         }         s.push(x);         while (!tmp.empty()) {             s.push(tmp.top());             tmp.pop();         }     }       // Removes the element from in front of queue.     void pop(void) {         s.pop();     }       // Get the front element.     int peek(void) {         return s.top();     }       // Return whether the queue is empty.     bool empty(void) {         return s.empty();     }   private:     stack<int> s; };

C++:

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class Queue { public:     // Push element x to the back of queue.     void push(int x) {         _new.push(x);     }           void shiftStack() {         if (_old.empty()) {             while (!_new.empty()) {                 _old.push(_new.top());                 _new.pop();             }         }     }       // Removes the element from in front of queue.     void pop(void) {         shiftStack();         if (!_old.empty()) _old.pop();     }       // Get the front element.     int peek(void) {         shiftStack();         if (!_old.empty()) return _old.top();         return 0;     }       // Return whether the queue is empty.     bool empty(void) {         return _old.empty() && _new.empty();     }   private:     stack<int> _old, _new; };

　　　 

类似题目：

[LeetCode] 225. Implement Stack using Queues 用队列来实现栈

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