用队列实现栈

思路

1. 入队列就是入栈
2. 出队列的时候，就是把前面size-1个队列中的元素先出，这样最后一个元素就成队首元素了，再把出去的元素再次入队列
3. 读栈顶元素，过程和第二步是一样的，就是弹出后，再把它入队列

typedef struct Node{int value;struct Node *next;
}Node;//头删+尾插typedef struct Queue{Node *front;  //链表第一个结点Node *rear;   //链表最后一个结点int size;	//数据个数
}Queue;//初始化
void QueueInit(Queue *queue){queue->front = NULL;queue->rear = NULL;queue->size = 0;
}
//销毁
void QueueDestroy(Queue *queue){Node *node, *next;for (node = queue->front; node != NULL; node = next){next = node->next;free(node);}queue->front = queue->rear = NULL;queue->size = 0;
}//Push,队尾插入，即链表的尾插
void QueuePush(Queue *queue, int val){Node *node = (Node*)malloc(sizeof(Node));node->value = val;node->next = NULL;if (queue->rear != NULL){queue->rear->next = node;queue->rear = node;}else{queue->front = queue->rear = node;}queue->size++;
}//Pop 队尾出列，即链表的头删
void QueuePop(Queue* queue){assert(queue->size > 0);queue->size--;Node *front = queue->front;queue->front = queue->front->next;free(front);if (queue->front == NULL){//处理只有一个结点的情况queue->rear = NULL;}
}
//Front 返回队首元素
int QueueFront(const Queue *queue){return queue->front->value;
}
//Rear  返回队尾元素
int QueueRear(const Queue *queue){return queue->rear->value;
}
//Empty 是否为空
int QueueEmpty(const Queue *queue){return queue->size == 0 ? 1 : 0;
}
//Size  
int QueueSize(const Queue *queue){return queue->size ;
}typedef struct{Queue queue;int maxSize;
}MyStack;/** Initialize your data structure here. */MyStack* myStackCreate() {MyStack *stack = (MyStack *)malloc(sizeof(MyStack));QueueInit(&(stack->queue));stack->maxSize = sizeof(MyStack);return stack;
}/** Push element x onto stack. */
void myStackPush(MyStack* obj, int x) {QueuePush(&(obj->queue), x);
}/** Removes the element on top of the stack and returns that element. */
int myStackPop(MyStack* obj) {int size = QueueSize(&(obj->queue));for (int i = 0; i < size - 1; i++){int val = QueueFront(&(obj->queue));QueuePop(&(obj->queue));QueuePush(&(obj->queue), val);}int val = QueueFront(&(obj->queue));QueuePop(&(obj->queue));return val;
}/** Get the top element. */
int myStackTop(MyStack* obj) {int size = QueueSize(&(obj->queue));for (int i = 0; i < size - 1; i++){int val = QueueFront(&(obj->queue));QueuePop(&(obj->queue));QueuePush(&(obj->queue), val);}int val = QueueFront(&(obj->queue));QueuePop(&(obj->queue));QueuePush(&(obj->queue),val);return val;
}/** Returns whether the stack is empty. */
bool myStackEmpty(MyStack* obj) {return QueueEmpty(&(obj->queue));
}void myStackFree(MyStack* obj) {QueueDestroy(&(obj->queue));free(obj);
}/*** Your MyStack struct will be instantiated and called as such:* MyStack* obj = myStackCreate();* myStackPush(obj, x);* int param_2 = myStackPop(obj);* int param_3 = myStackTop(obj);* bool param_4 = myStackEmpty(obj);* myStackFree(obj);
*/

用栈实现队列

思路

1. 要用两个栈实现队列
2. 一个为左边栈，一个为右边栈，入队列就相当于给左边栈压栈，而出队列，则是看右边栈空不空，如果为空，则将左边栈的所有元素全部依次压入右边栈，然后取右边栈顶元素，出栈（也就是出队列）
3. 返回队首元素，过程跟第二步一样，唯一区别就是不用出栈，直接返回就行
4. 两边栈都为空时，队列为空

class MyQueue {
public:/** Initialize your data structure here. */MyQueue() {}/** Push element x to the back of queue. */void push(int x) {left.push(x);}/** Removes the element from in front of queue and returns that element. */int pop() {if (right.empty()){while (!left.empty()){int top = left.top();left.pop();right.push(top);}}int top = right.top();right.pop();return top;}/** Get the front element. */int peek() {if (right.empty()){while (!left.empty()){int top = left.top();left.pop();right.push(top);}}int top = right.top();return top;}/** Returns whether the queue is empty. */bool empty() {return left.empty() && right.empty();}stack<int>left;stack<int>right;
};/*** 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();* bool param_4 = obj->empty();*/