1.為什麼建立二叉樹時使用二級指標？

C都是值傳遞，只傳指標相當於把外部指標的地址傳入了而已，在函式內部你又建立了一個指標，也是指向那個地址，操作完後，其實只是改變了你建立的指標，並未改變外部的指標。

如同這一個函式，並未交換兩個值。

你要改變的是指標的地址，那就要用到指標的指標，即二重指標

void change(int a, int b)
{
  int temp;
  temp = a;
  a = b;
  b = temp;
}

1.有同學問如何終止輸入？

輸入時必須將所有葉子節點填充，如同ABD##C##EF##H##,ABC##D##E##這樣完整的樹

二叉樹的建立本質上其實就是遍歷

先序遍歷：跟，左，右

中序遍歷：左，跟，右

後序遍歷：左，右，跟

層次遍歷：從根節點開始，依次往下從左到右

其他的操作不難，大家看看原始碼即可。

遞迴實現二叉樹並不難

#include<stdio.h>
#include<stdlib.h>

typedef struct binTree {
  char data;
  struct binTree * left;
  struct binTree * right;
}* BinTree, Node;

void createTree(BinTree * tree) {
  char ch;
  ch = getchar();

  if (ch == '#') {
    *tree = NULL;
  } else {
    *tree = (BinTree) malloc (sizeof(Node));
    (*tree)->data = ch;
    (*tree)->left = NULL;
    (*tree)->right = NULL;
    createTree(&(*tree)->left);
    createTree(&(*tree)->right);
  }
}

void preOrder(BinTree tree) {
  if (tree) {
    printf("%c", tree->data);
    preOrder(tree->left);
    preOrder(tree->right);
  }
}

void midOrder(BinTree tree) {
  if (tree) {
    midOrder(tree->left);
    printf("%c", tree->data);
    midOrder(tree->right);
  }
}

void postOrder(BinTree tree) {
  if (tree) {
    postOrder(tree->left);
    postOrder(tree->right);
    printf("%c", tree->data);
  }
}

void insertLeft(BinTree tree, char ch) {
  BinTree p, node;

  if (!tree) {
    printf("樹為空\n");
    return ;
  }

  p = tree->left;
  node = (BinTree)malloc(sizeof(Node));
  node->right = NULL;
  node->data = ch;
  node->left = p;
  tree->left = node;

}

void insertRight(BinTree tree, char ch) {
  BinTree p, node;

  if (!tree) {
    printf("樹為空\n");
    return ;
  }
  p = tree->right;
  node = (BinTree)malloc(sizeof(Node));
  node->data = ch;
  node->left = NULL;
  node->right = p;
  tree->right = node;
}

int countLeaf(BinTree tree) {
  if (tree == NULL) {
    return 0;
  }

  if (!(tree->left) && !(tree->right)) {
    return 1;
  } else {
    return countLeaf(tree->left) + countLeaf(tree->right);
  }
}

int countNode(BinTree tree) {
  if (!tree) {
    return 0;
  } else {
    return countNode(tree->left) + countNode(tree->right) + 1;
  }
}

int depthTree(BinTree tree) {
  int left, right;

  if (!tree) {
    return 0;
  }

  left = depthTree(tree->left);
  right = depthTree(tree->right);
  return (left > right)?(1+left):(1+right);
}

void printNode(BinTree tree, int level) {
    if (!tree || level < 1) {
        return ;
    }

    if (level == 1) {
        printf("%c", tree->data);
    }

    printNode(tree->left, level - 1);
    printNode(tree->right, level - 1);
}

void printAtLevel(BinTree tree) {
    int depth = depthTree(tree);
    int i;

    if (!tree) {
        printf("樹為空");
    }

    for (i = 1; i <= depth; i++) {
        printNode(tree, i);
        printf("\n");
    }

}

int main(void)
{
  BinTree tree;
  char ch;
  int num;

  printf("輸入二叉樹，#為空:");
  createTree(&tree);

  printf("1.先序遍歷   2.中序遍歷   3.後序遍歷   4.層次遍歷\n");
  printf("5.葉子節點數   6.節點數   7.左節點插入   8.右節點插入   9.樹深度   10.退出\n");
  while (scanf("%d", &num) != EOF && num != 10) {
    switch (num) {
      case 1:preOrder(tree);putchar('\n');break;
      case 2:midOrder(tree);putchar('\n');break;
      case 3:postOrder(tree);putchar('\n');break;
      case 4:printAtLevel(tree);putchar('\n');break;
      case 5:printf("葉子節點數:%d\n", countLeaf(tree));break;
      case 6:printf("節點數:%d\n", countNode(tree));break;
      case 7:printf("請輸入插入節點:");getchar();scanf("%c", &ch);insertLeft(tree, ch);
             printf("插入成功\n\n");break;
      case 8:printf("請輸入插入節點:");getchar();scanf("%c", &ch);insertRight(tree, ch);
             printf("插入成功\n\n");break;
      case 9:printf("樹深度為:%d\n\n", depthTree(tree));break;
      default:printf("請輸入合法操作！\n");break;
    }
  }
  return 0;
}