数据结构（10）栈和队列算法题

一、用队列实现栈

1、题目描述

https://leetcode.cn/problems/implement-stack-using-queues

 2、算法分析

入栈：往不为空的队列中插入数据。

出栈：把不为空的队列中前size-1个数据挪到另一个队列中，再将最后一个数据出队。

取栈顶元素：取不为空队列中队尾结点的数据。

3、参考代码

typedef int QDataType;
//队列结点的结构
typedef struct QueueNode
{
	QDataType data;
	struct QueueNode* next;
}QueueNode;
//队列的结构
typedef struct Queue
{
	QueueNode* phead;
	QueueNode* ptail;
	int size; //队列中有效数据个数
}Queue;
//初始化
void QueueInit(Queue* pq)
{
	assert(pq);
	pq->phead = pq->ptail = NULL;
	pq->size = 0;
}
//销毁队列
void QueueDestroy(Queue* pq)
{
	assert(pq);
	QueueNode* pcur = pq->phead;
	while (pcur)
	{
		QueueNode* next = pcur->next;
		free(pcur);
		pcur = next;
	}
	pq->phead = pq->ptail = NULL;
	pq->size = 0;
}
//入队——队尾
void QueuePush(Queue* pq, QDataType x)
{
	assert(pq);
	QueueNode* newnode = (QueueNode*)malloc(sizeof(QueueNode));
	if (newnode == NULL)
	{
		perror("malloc fail!");
		exit(1);
	}
	newnode->data = x;
	newnode->next = NULL;
	//队列为空
	if (pq->phead == NULL)
	{
		pq->phead = pq->ptail = newnode;
	}
	else
	{
		//队列非空
		pq->ptail->next = newnode;
		pq->ptail = pq->ptail->next;
	}
	pq->size++;
}
//队列判空
bool QueueEmpty(Queue* pq)
{
	assert(pq);
	return pq->phead == NULL;
}
//出队——队头
void QueuePop(Queue* pq)
{
	assert(!QueueEmpty(pq));
	//只有一个结点，phead和ptail都要置为空
	if (pq->phead == pq->ptail)
	{
		free(pq->phead);
		pq->phead = pq->ptail = NULL;
	}
	else
	{
		QueueNode* next = pq->phead->next;
		free(pq->phead);
		pq->phead = next;
	}
	pq->size--;
}
//取队头数据
QDataType QueueFront(Queue* pq)
{
	assert(!QueueEmpty(pq));
	return pq->phead->data;
}
//取队尾数据
QDataType QueueBack(Queue* pq)
{
	assert(!QueueEmpty(pq));
	return pq->ptail->data;
}
//队列有效元素个数
int QueueSize(Queue* pq)
{
	assert(pq);
	return pq->size;
}

/////////////////////////以上是队列结构和方法的实现/////////////////////
typedef struct 
{
    Queue q1;
    Queue q2;
} MyStack;


MyStack* myStackCreate() 
{
    MyStack* pst = (MyStack*)malloc(sizeof(MyStack));
    QueueInit(&pst->q1);
    QueueInit(&pst->q2);

    return pst;
}

void myStackPush(MyStack* obj, int x) 
{
    //往不为空的队列中插入数据
    if(!QueueEmpty(&obj->q1))
    {
        QueuePush(&obj->q1, x);
    }
    else
    {
        QueuePush(&obj->q2, x);
    }
}

int myStackPop(MyStack* obj) 
{
    //将不为空的队列中前size-1个数据挪到另一个队列中
    //再将最后一个数据出队列
    Queue* emp = &obj->q1;
    Queue* nonEmp = &obj->q2;
    if(QueueEmpty(&obj->q2))
    {
        nonEmp = &obj->q1;
        emp = &obj->q2;
    }
    while(QueueSize(nonEmp) > 1)
    {
        int front = QueueFront(nonEmp);
        QueuePush(emp, front);
        QueuePop(nonEmp);
    }
    int top = QueueFront(nonEmp);
    QueuePop(nonEmp);
    return top;
}
//取栈顶
int myStackTop(MyStack* obj) 
{
    //找不为空队列中的队尾数据
    if(!QueueEmpty(&obj->q1))
    {
        return QueueBack(&obj->q1);
    }
    else
    {
        return QueueBack(&obj->q2);
    }
}

bool myStackEmpty(MyStack* obj) 
{
    return QueueEmpty(&obj->q1) && QueueEmpty(&obj->q2);
}
//销毁
void myStackFree(MyStack* obj) 
{
    QueueDestroy(&obj->q1);
    QueueDestroy(&obj->q2);

    free(obj);
    obj = NULL;
}

/**
 * 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);
*/

二、用栈实现队列

1、题目描述

https://leetcode.cn/problems/implement-queue-using-stacks

2、算法分析 

入队：往pushST中插入数据。

出队：如果popST不为空，直接出数据；否则将popST中的数据导入到popST中再出数据。

取队头元素：逻辑同出队操作，但是这里只取数据，不删除数据。

3、参考代码

//定义栈的结构
typedef int STDataType;
typedef struct Stack
{
	STDataType* arr;
	int top;       //指向栈顶的位置
	int capacity;  //栈的容量
}ST;
//初始化
void StackInit(ST* ps)
{
	ps->arr = NULL;
	ps->top = ps->capacity = 0;
}
//入栈——栈顶
void StackPush(ST* ps, STDataType x)
{
	assert(ps);
	if (ps->top == ps->capacity)
	{
		//增容
		int newCapacity = ps->capacity == 0 ? 4 : 2 * ps->capacity;
		STDataType* tmp = (STDataType*)realloc(ps->arr, newCapacity * sizeof(STDataType));
		if (tmp == NULL)
		{
			perror("realloc fail!");
			exit(1);
		}
		ps->arr = tmp;
		ps->capacity = newCapacity;
	}
	ps->arr[ps->top++] = x;
}
//栈是否为空
bool StackEmpty(ST* ps)
{
	assert(ps);
	return ps->top == 0;
}
//出栈——栈顶
void StackPop(ST* ps)
{
	assert(!StackEmpty(ps));
	ps->top--;
}
//取栈顶元素
STDataType StackTop(ST* ps)
{
	assert(!StackEmpty(ps));
	return ps->arr[ps->top - 1];
}
//获取栈中有效元素个数
int StackSize(ST* ps)
{
	return ps->top;
}
//销毁
void StackDestroy(ST* ps)
{
	if (ps->arr)
		free(ps->arr);
	ps->arr = NULL;
	ps->capacity = ps->top = 0;
}
/////////////////////以上是栈的结构和实现///////////////////////

typedef struct 
{
    ST pushST;
    ST popST;
} MyQueue;


MyQueue* myQueueCreate() 
{
    MyQueue* pq = (MyQueue*)malloc(sizeof(MyQueue));
    StackInit(&pq->pushST);
    StackInit(&pq->popST);

    return pq;
}

void myQueuePush(MyQueue* obj, int x) 
{
    //往pushST中插入数据
    StackPush(&obj->pushST, x);
}
// 检查popST是否为空
// 1)不为空，直接出popST的栈顶
// 2)为空，pushST中的数据导入到popST中，再出popST栈顶
int myQueuePop(MyQueue* obj) 
{
    if(StackEmpty(&obj->popST))
    {
        //导数据
        while(!StackEmpty(&obj->pushST))
        {
            int data = StackTop(&obj->pushST);
            StackPush(&obj->popST, data);
            StackPop(&obj->pushST);
        }
    }
    int top = StackTop(&obj->popST);
    StackPop(&obj->popST);
    return top;
}

int myQueuePeek(MyQueue* obj) 
{
    if(StackEmpty(&obj->popST))
    {
        //导数据
        while(!StackEmpty(&obj->pushST))
        {
            int data = StackTop(&obj->pushST);
            StackPush(&obj->popST, data);
            StackPop(&obj->pushST);
        }
    }
    int top = StackTop(&obj->popST);
    return top;
}

bool myQueueEmpty(MyQueue* obj) 
{
    return StackEmpty(&obj->pushST) && StackEmpty(&obj->popST);
}

void myQueueFree(MyQueue* obj) 
{
    StackDestroy(&obj->pushST);
    StackDestroy(&obj->popST);
    
    free(obj);
    obj = NULL;
}

/**
 * Your MyQueue struct will be instantiated and called as such:
 * MyQueue* obj = myQueueCreate();
 * myQueuePush(obj, x);
 
 * int param_2 = myQueuePop(obj);
 
 * int param_3 = myQueuePeek(obj);
 
 * bool param_4 = myQueueEmpty(obj);
 
 * myQueueFree(obj);
*/