力扣 232. 用栈实现队列(C语言实现)

1.解题思路

利用两个栈，设栈s1为入栈,s2为出栈，则当s2为空时，出队列只能将s1的后N-1项挪到s2后剩下的就为出栈的项，但如果s2不为空那就说明此时s2栈中的元素就已经是按照队列的顺序排好了，直接出栈即可.

2.代码实现

栈的实现代码:

typedef int STDataType;
typedef struct Stack
{
	STDataType* _a;
	int _top;		// 栈顶
	int _capacity;  // 容量 
}Stack;
void StackInit(Stack* ps)
{
	assert(ps);
	ps->_a = NULL;
	ps->_capacity = 0;
	ps->_top = 0;
}
void StackPush(Stack* pst, STDataType x)
{
	assert(pst);

	if (pst->_top == pst->_capacity)
	{
		int newcapacity = pst->_capacity == 0 ? 4 : pst->_capacity * 2;
		STDataType* tmp = (STDataType*)realloc(pst->_a, sizeof(STDataType) * newcapacity);
		if (tmp == NULL)
		{
			perror("realloc fail");
			return;
		}

		pst->_a = tmp;
		pst->_capacity = newcapacity;
	}

	pst->_a[pst->_top] = x;
	pst->_top++;
}
void StackPop(Stack* ps)
{
	assert(ps->_top > 0);
	ps->_top--;

}
STDataType StackTop(Stack* ps)
{

	return ps->_a[ps->_top - 1];

}
int StackSize(Stack* ps)
{
	return ps->_top;

}
bool StackEmpty(Stack* ps)
{
	return ps->_top == 0;

}
void StackDestroy(Stack* ps)
{
	free(ps->_a);
	ps->_capacity = 0;
	ps->_top = 0;

}

解题代码:

typedef struct {
	Stack s1;
	Stack s2;
} MyQueue;


MyQueue* myQueueCreate() {

	MyQueue* m = (MyQueue*)malloc(sizeof(MyQueue));
	StackInit(&m->s1);
	StackInit(&m->s2);
}

void myQueuePush(MyQueue* obj, int x) {

	StackPush(&obj->s1, x);

}

int myQueuePop(MyQueue* obj) {
	if (!StackEmpty(&obj->s2))
	{
		int tmp = StackTop(&obj->s2);
		StackPop(&obj->s2);
		return tmp;
	}
	else
	{
		while (StackSize(&obj->s1) > 1)
		{
			int tmp = StackTop(&obj->s1);
			StackPush(&obj->s2, tmp);
			StackPop(&obj->s1);
		}
		int tmp1 = StackTop(&obj->s1);
		StackPop(&obj->s1);
		return tmp1;
	}
}


int myQueuePeek(MyQueue* obj) {
	if (!StackEmpty(&obj->s1))
	{
		return obj->s1._a[0];
	}
	else
	{
		return obj->s2._a[obj->s2._top - 1];
	}

}

bool myQueueEmpty(MyQueue* obj) {

	if (StackEmpty(&obj->s1) && StackEmpty(&obj->s2))
		return true;
	else
		return false;

}

void myQueueFree(MyQueue* obj) {

	StackDestroy(&obj->s1);
	StackDestroy(&obj->s2);

}

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