二叉树的三种遍历方法：
前序遍历：根结点 —> 左子树 —> 右子树

中序遍历：左子树—> 根结点 —> 右子树

后序遍历：左子树 —> 右子树 —> 根结点

下面是三种遍历的代码和计算树的大小，计算叶子的个数，树的高度和计算k层结点的个数，都是递归思想

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>typedef int BTDatatype;typedef struct BinaryTreeNode {BTDatatype data;struct BinaryTreeNode* left;struct BinaryTreeNode* right;
}BTNode;BTNode* CreatNode(BTDatatype x)//创建树的结点
{BTNode* node = (BTNode*)malloc(sizeof(BTNode));assert(node);node->data = x;node->left = NULL;node->right = NULL;return node;
}BTNode* CreatTree()//手动创建一个树
{BTNode* node1 = CreatNode(1);BTNode* node2 = CreatNode(2);BTNode* node3 = CreatNode(3);BTNode* node4 = CreatNode(4);BTNode* node5 = CreatNode(5);BTNode* node6 = CreatNode(6);BTNode* node7 = CreatNode(7);node1->left = node2;node1->right = node4;node2->left = node3;node4->left = node5;node4->right = node6;node5->right = node7;return node1;
}void PrevOrder(BTNode* root)//前序遍历
{if (root == NULL){printf("N ");return;}printf("%d ", root->data);PrevOrder(root->left);PrevOrder(root->right);
}void INOrder(BTNode* root)//中序遍历
{if (root == NULL){printf("N ");return;}INOrder(root->left);printf("%d ", root->data);INOrder(root->right);
}void TailOrder(BTNode* root)//后序遍历
{if (root == NULL){printf("N ");return;}TailOrder(root->left);TailOrder(root->right);printf("%d ", root->data);
}int TreeSize(BTNode* root)//树的大小
{return (root == NULL) ? 0 : TreeSize(root->left) + TreeSize(root->right) + 1;
}int TreeLeafSize(BTNode* root)//叶子的个数
{if (root == NULL)return 0;if (root->left == NULL && root->right == NULL)return 1;return TreeLeafSize(root->left) + TreeLeafSize(root->right);
}int TreeHigh(BTNode* root)//树的高度
{if (root == NULL)return 0;int LeftHigh = TreeHigh(root->left);int RightHigh = TreeHigh(root->right);return  LeftHigh > RightHigh ? LeftHigh + 1 : RightHigh + 1;
}int numk(BTNode* root, int k)//计算k层结点的个数
{assert(k > 0);if (root == NULL)return 0;if (k == 1){return 1;}return  numk(root->left, k - 1) + numk(root->right, k - 1);
}int main()
{BTNode* root = CreatTree();printf("前序遍历：");PrevOrder(root);printf("\n");printf("中序遍历：");INOrder(root);printf("\n");printf("后序遍历：");TailOrder(root);printf("\n");printf("树的大小：");printf("%d", TreeSize(root));printf("\n");printf("叶子的个数：");printf("%d", TreeLeafSize(root));printf("\n");printf("树的高度：");printf("%d", TreeHigh(root));printf("\n");int k = 0;scanf("%d", &k);printf("第%d层的结点个数为：", k);printf("%d", numk(root, k));printf("\n");return 0;
}