层序是采用队列的方式来遍历的

就比如说上面这颗树

他层序的就是：1 24 356

 

void LevelOrder(BTNode* root)
{
	Que q;
	QueueInit(&q);
	if (root)
	{
		QueuePush(&q, root);
	}
	while (!QueueEmpty(&q))
	{
		BTNode* front = QueueFront(&q);
		QueuePop(&q);
		printf("%d ", front->val);

		if (front->left)
		{
			QueuePush(&q, front->left);
		}
		if (front->right)
		{
			QueuePush(&q, front->right);
		}
	}

}

打印NULL ，这里front也会把NULL带进去

void LevelOrder(BTNode* root)
{
	Que q;
	QueueInit(&q);
	if (root)
	{
		QueuePush(&q, root);
	}
	while (!QueueEmpty(&q))
	{
		BTNode* front = QueueFront(&q);
		QueuePop(&q);
		
		if (front)
		{
			printf("%d ", front->val);
		
			QueuePush(&q, front->left);
			QueuePush(&q, front->right);
			
		}
		else
		{
			printf("N ");
		}
		
		
	}

}

 完整代码（栈和队列是复制前几篇的代码）

#pragma once
#include<stdio.h>
#include<stdlib.h>
#include<assert.h>
typedef int BinTreeType;
struct BinTreeNode
{
	struct BinTreeNode* left;
	struct BinTreeNode* right;
	BinTreeType val;

}; 
typedef struct BinTreeNode BTNode;
 
BTNode* BuyBTNode(BinTreeType val);
BTNode* CreateTree();
void PreOrder(BTNode* root);
void InOrder(BTNode* root);
void PostOrder(BTNode* root);
int TreeSize(BTNode* root);
int MaxDepth(BTNode* root);
int TreeLevel(BTNode* root, int k);
BTNode* TreeFind(BTNode* root, int x);
int BinaryTreeLeafSize(BTNode* root);

 

#pragma once
#include<stdio.h>
#include<stdlib.h>
#include<assert.h>
#include<stdbool.h>
#include"BinTree.h"
typedef BTNode* QueueData;
struct QueueNode
{
	QueueData val;
	struct QueueNode* next;
};
typedef struct QueueNode QNode;
typedef struct Queue
{
	QNode* phead;
	QNode* ptail;
	int size;
}Que;
void QueueInit(Que* pq);//队列初始化
void QueueDestroy(Que* pq);//队列销毁

void QueuePush(Que* pq,QueueData x);//入队列
void QueuePop(Que* pq);//出队列

QueueData QueueBack(Que* pq);
QueueData QueueFront(Que* pq);
bool QueueEmpty(Que* pq);
int QueueSize(Que* pq);

#define _CRT_SECURE_NO_WARNINGS 1
#include"BinTree.h"
BTNode* BuyBTNode(BinTreeType val)
{
	BTNode* newnode = (BTNode*)malloc(sizeof(BTNode));
	if (newnode == NULL)
	{
		perror("malloc fail!");
		exit(1);
	}
	newnode->val = val;
	newnode->left = NULL;
	newnode->right = NULL;
	return newnode;
}
BTNode* CreateTree()
{
	BTNode* n1 = BuyBTNode(1);
	BTNode* n2 = BuyBTNode(2);
	BTNode* n3 = BuyBTNode(3);
	BTNode* n4 = BuyBTNode(4);
	BTNode* n5 = BuyBTNode(5);
	BTNode* n6 = BuyBTNode(6);
	n1->left = n2;
	n1->right = n4;
	n2->left = n3;
	n2->right = NULL;
	n3->left = NULL;
	n3->right = NULL;
	n4->left = n5;
	n4->right = n6;
	n5->left = NULL;
	n5->right = NULL;
	n6->left = NULL;
	n6->right = NULL;
	return n1;
}
void PreOrder(BTNode* root)
{
	if (root == NULL)
	{
		printf("NULL ");
		return ;
	}
	printf("%d ", root->val);
	PreOrder(root->left);
	PreOrder(root->right);
}
void InOrder(BTNode* root)
{
	if (root == NULL)
	{
		printf("NULL ");
		return;
	}
	InOrder(root->left);
	printf("%d ", root->val);
	InOrder(root->right);
}
void PostOrder(BTNode* root)
{
	if (root == NULL)
	{
		printf("NULL ");
		return;
	}
	PostOrder(root->left);
	PostOrder(root->right);
	printf("%d ", root->val);
}
int TreeSize(BTNode* root)
{
	if (root == NULL)
	{
		return 0;
	}
	else
	{
		return TreeSize(root->left) + TreeSize(root->right)+1;
	}

}

int MaxDepth(BTNode* root)
{
	if (root == NULL)
	{
		return 0;
	}
	int leftDepth = MaxDepth(root->left);
	int rightDepth = MaxDepth(root->left);
	if (leftDepth > rightDepth)
	{
		return leftDepth + 1;
	}
	else
	{
		return rightDepth + 1;
	}
}
int TreeLevel(BTNode* root, int k)
{
	assert(k > 0);
	
	if (root == NULL)
	{
		return 0;
	}

	if (k == 1)
	{
		return 1;
	}

	return TreeLevel(root->left, k - 1) + TreeLevel(root->right, k - 1);
}
BTNode* TreeFind(BTNode* root, int x)
{
	if (root == NULL)
	{
		return NULL;
	}

	if (root->val == x)
	{
		return root;
	}

	BTNode* ret1 = TreeFind(root->left, x);

	if (ret1)
	{
		return ret1;
	}
	return TreeFind(root->right, x); 

}
int BinaryTreeLeafSize(BTNode* root)
{
	if (root == NULL)
	{
		return 0;
	}

	if (BinaryTreeLeafSize(root->left) == NULL && BinaryTreeLeafSize(root->right) == NULL)
	{
		return 1;	
	} 

	return BinaryTreeLeafSize(root->left) + BinaryTreeLeafSize(root->right);
}

 

#define _CRT_SECURE_NO_WARNINGS 1
#include"queue.h"
void QueueInit(Que* pq)
{
	assert(pq);
	pq->phead = NULL;
	pq->ptail = NULL;
	pq->size = 0;
}
void QueueDestroy(Que* pq)
{
	QNode* pcur = pq->phead;
	while (pcur)
	{
		QNode* next = pcur->next;
		free(pcur);
		pcur = next;
	}
	pq->phead = pq->ptail = NULL;
	pq->size = 0;	
}

void QueuePush(Que* pq,QueueData x)
{
	assert(pq);
	QNode* newnode = (Que*)malloc(sizeof(QNode));
	if (newnode == NULL)
	{
		perror("malloc fail");
		exit(1);
	}
	newnode->val = x;
	newnode->next = NULL;
	if (pq->ptail)
	{
		pq->ptail->next = newnode;
		pq->ptail = newnode;
	}
	else
	{
		pq->phead = pq->ptail = newnode;
	}
	pq->size++;
}
void QueuePop(Que* pq)
{
	assert(pq->phead != NULL);

	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--;
}

QueueData QueueFront(Que* pq)
{
	assert(pq!=NULL);

	assert(pq->phead != NULL);
	return pq->phead->val;
}
QueueData QueueBack(Que* pq)
{
	assert(pq!=NULL);

	assert(pq->ptail != NULL);
	return pq->ptail->val;
}
bool QueueEmpty(Que* pq)
{
	return pq->phead == NULL;
}
int QueueSize(Que* pq)
{
	assert(pq);
	return pq->size;

}

 

#define _CRT_SECURE_NO_WARNINGS 1
#include"BinTree.h"
#include"queue.h"
void LevelOrder(BTNode* root)
{
	Que q;
	QueueInit(&q);
	if (root)
	{
		QueuePush(&q, root);
	}
	while (!QueueEmpty(&q))
	{
		BTNode* front = QueueFront(&q);
		QueuePop(&q);
		
		if (front)
		{
			printf("%d ", front->val);
		
			QueuePush(&q, front->left);
			QueuePush(&q, front->right);
			
		}
		else
		{
			printf("N ");
		}
		
		
	}

}
int main()
{
	BTNode* root= CreateTree();
	//PreOrder(root);
	printf("\n");
	
	LevelOrder(root);

}