二叉树的遍历方式主要有：先序遍历、中序遍历、后序遍历、层次遍历。先序、中序、后序其实指的是父节点被访问的次序。若在遍历过程中，父节点先于它的子节点被访问，就是先序遍历；

#include<stdio.h>
#include<malloc.h>
 
#define QUEUE_SIZE 5
 
typedef struct BTNode {
	char element;
	BTNode* left;
	BTNode* right;
}BTNode,*BTNodePtr;
 
typedef struct BTNodePtrQueue {
	BTNodePtr* nodePtrs;
	int front;
	int rear;
}BTNodePtrQueue, *QueuePtr;
 
QueuePtr initQueue() {
	QueuePtr resultQueuePtr = (QueuePtr)malloc(sizeof(struct BTNodePtrQueue));
	resultQueuePtr->nodePtrs = (BTNodePtr*)malloc(QUEUE_SIZE * sizeof(BTNodePtr));
	resultQueuePtr->front = 0;
	resultQueuePtr->rear = 1;
	resultQueuePtr; 
}
 
bool isQueueEmpty(QueuePtr paraQueuePtr) {
	if ((paraQueuePtr->front + 1) % QUEUE_SIZE == paraQueuePtr->rear) {
		return true;
	}
	return false;
}
 
 
void enqueue(QueuePtr paraQueuePtr, BTNodePtr paraBTNodePtr) {
	printf("front = %d, rear = %d\n",paraQueuePtr->front,paraQueuePtr->rear);
	if ((paraQueuePtr->rear + 1) % QUEUE_SIZE == paraQueuePtr->front % QUEUE_SIZE) {
		printf("Error, trying to enqueue %c. queue full.\n",paraBTNodePtr->element);
		return;
	}
	paraQueuePtr->nodePtrs[paraQueuePtr->rear] = paraBTNodePtr;
	paraQueuePtr->rear = (paraQueuePtr->rear + 1) % QUEUE_SIZE;
	printf("enqueue %c ends.\n",paraBTNodePtr->element);
}
 
BTNodePtr dequeue(QueuePtr paraQueuePtr) {
	if (isQueueEmpty(paraQueuePtr)) {
		printf("Error, empty queue\n");
		return NULL;
	}
	paraQueuePtr->front = (paraQueuePtr->front + 1) % QUEUE_SIZE;
	
	printf("dequeue %c ends.\n",paraQueuePtr->nodePtrs[paraQueuePtr->front]->element);
	return paraQueuePtr->nodePtrs[paraQueuePtr->front];
}
 
BTNodePtr constructBTNode(char paraChar) {
	BTNodePtr resultPtr = (BTNodePtr)malloc(sizeof(BTNode));
	resultPtr->element = paraChar;
	resultPtr->right = NULL;
	resultPtr->left = NULL;
	return resultPtr; 
}
 
BTNodePtr stringToBTree(char* paraString) {
	int i;
	char ch;
	
	QueuePtr tempQueuePtr = initQueue();
	
	BTNodePtr resultHeader;
	BTNodePtr tempParent, tempLeftChild, tempRightChild;
	i = 0;
	ch = paraString[i];
	resultHeader = constructBTNode(ch);
	enqueue(tempQueuePtr, resultHeader);
	
	while (!isQueueEmpty(tempQueuePtr)) {
		tempParent = dequeue(tempQueuePtr);
		
		i++;
		ch = paraString[i];
		if (ch == '#') {
			tempParent->left = NULL;
		}
		else {
			tempLeftChild = constructBTNode(ch);
			enqueue(tempQueuePtr, tempLeftChild);
			tempParent->left = tempLeftChild; 
		}
		
		i++;
		ch = paraString[i];
		if (ch == '#') {
			tempParent->right = NULL;
		}
		else {
			tempRightChild = constructBTNode(ch);
			enqueue(tempQueuePtr, tempRightChild);
			tempParent->right = tempRightChild;
		}
	}
	
	return resultHeader; 
}
 
void levelWise(BTNodePtr paraTreePtr) {
	char tempString[100];
	int i = 0;
	QueuePtr tempQueuePtr = initQueue();
	BTNodePtr tempNodePtr;
	enqueue(tempQueuePtr, paraTreePtr);
	while (!isQueueEmpty(tempQueuePtr)) {
		tempNodePtr = dequeue(tempQueuePtr);
		
		tempString[i] = tempNodePtr->element;
		i++;
		
		if (tempNodePtr->left != NULL) {
			enqueue(tempQueuePtr, tempNodePtr->left);
		}  
		if (tempNodePtr->right != NULL) {
			enqueue(tempQueuePtr, tempNodePtr->right);
		}
	}
	
	tempString[i] = ' ';
	
	printf("LevelWise:-> %s\n",tempString);
}
 
void preorder(BTNodePtr tempPtr) {
	if (tempPtr == NULL) {
		return;
	}
	
	printf("%c",tempPtr->element);
	preorder(tempPtr->left);
	preorder(tempPtr->right);
}
 
void inorder(BTNodePtr tempPtr) {
	if (tempPtr == NULL) {
		return;
	}
	
	inorder(tempPtr->left);
	printf("%c",tempPtr->element);
	inorder(tempPtr->right);
}
 
void postorder(BTNodePtr tempPtr) {
	if (tempPtr == NULL) {
		return;
	}
	
	postorder(tempPtr->left);
	postorder(tempPtr->right);
	printf("%c",tempPtr->element);
}
 
int main() {
	BTNodePtr tempHeader;
	tempHeader = constructBTNode('c');
	printf("There is only one node. preorder visit:");
	preorder(tempHeader);
	printf("\r\n");
	
	char* tempString = "acde#bf######";
	
	tempHeader = stringToBTree(tempString);
	printf("Preorder:-> ");
	preorder(tempHeader);
	printf("\r\n");
	printf("Inorder:-> ");
	inorder(tempHeader);
	printf("\r\n");
	printf("Postorder:-> ");
	postorder(tempHeader);
	printf("\r\n");
	printf("LevelWise: \n");
	levelWise(tempHeader);
	printf("\r\n");
	
	return 1;
}