用两个队列（先进先出）实现一个栈（后进先出）
题目链接：https://leetcode.cn/problems/implement-stack-using-queues/description/
1.C语言首先要造一个队列出来
2.两个队列实现栈，始终保持一个队列为空，一个队列非空的状态
3.根据栈先进先出的特性，入栈要把数据放入非空队列中
4.取栈顶元素则取非空队列的队尾即可
5.出栈，并返回栈顶元素的值，则需要把非空队列中n个数据的前n-1个数据导入空队列中，剩下的唯一一个数据就是要出栈的数据。
typedef int QDataType;
typedef struct QueueNode
{QDataType val;struct QueueNode* next;
}QNode;
typedef struct Queue
{QNode* phead;QNode* ptail;int size;
}Queue;//初始化
void QueueInit(Queue* pq)
{assert(pq);pq->phead = pq->ptail = NULL;pq->size = 0;
}
//销毁
void QueueDestroy(Queue* pq)
{assert(pq);QNode* cur = pq->phead;while (cur){QNode* next = cur->next;free(cur);cur = next;}pq->phead = pq->ptail = NULL;pq->size = 0;
}
//入队列
void QueuePush(Queue* pq, QDataType x)
{assert(pq);//在队尾入，尾插//创建新节点QNode* newnode = (QNode*)malloc(sizeof(QNode));if (newnode == NULL){perror("malloc fail");return;}newnode->val = x;newnode->next = NULL;//空链表if (pq->phead==NULL){pq->phead = pq->ptail = newnode;}else {pq->ptail->next = newnode;pq->ptail = newnode;}pq->size++;
}
//出队列
void QueuePop(Queue* pq)
{assert(pq);//零个节点assert(pq->phead);//一个节点if (pq->phead->next == NULL){free(pq->phead);pq->phead = pq->ptail = NULL;}else{QNode* next = pq->phead->next;free(pq->phead);pq->phead = next;}pq->size--;
}
//判空
bool QueueEmpty(Queue* pq)
{assert(pq);return pq->size == 0;
}
//取队头
QDataType QueueFront(Queue* pq)
{assert(pq);assert(pq->phead);return pq->phead->val;
}
//取队尾
QDataType QueueBack(Queue* pq)
{assert(pq);assert(pq->ptail);return pq->ptail->val;
}
//队列长度
size_t QueueLength(Queue* pq)
{assert(pq);return pq->size;
}typedef struct {Queue obj1;Queue obj2;
} MyStack;MyStack* myStackCreate() {MyStack* p=(MyStack*)malloc(sizeof(MyStack));QueueInit(&p->obj1);QueueInit(&p->obj2);return p;
}void myStackPush(MyStack* obj, int x) {//入栈在非空队列if(QueueEmpty(&obj->obj1)){QueuePush(&obj->obj2,x);}else{QueuePush(&obj->obj1,x);}
}int myStackPop(MyStack* obj) {Queue* EmptyQ=&obj->obj1;Queue* NoEmptyQ=&obj->obj2;if(!QueueEmpty(&obj->obj1)){EmptyQ=&obj->obj2;NoEmptyQ=&obj->obj1;}while(QueueLength(NoEmptyQ)>1){int front=QueueFront(NoEmptyQ);QueuePush(EmptyQ,front);QueuePop(NoEmptyQ);}int front=QueueFront(NoEmptyQ);QueuePop(NoEmptyQ);return front;
}int myStackTop(MyStack* obj) {Queue* EmptyQ=&obj->obj1;Queue* NoEmptyQ=&obj->obj2;if(!QueueEmpty(EmptyQ)){EmptyQ=&obj->obj2;NoEmptyQ=&obj->obj1;}return QueueBack(NoEmptyQ);
}bool myStackEmpty(MyStack* obj) {return QueueEmpty(&obj->obj1)&&QueueEmpty(&obj->obj2);
}void myStackFree(MyStack* obj) {QueueDestroy(&obj->obj1);QueueDestroy(&obj->obj2);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);
*/
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