二叉搜身树（BST）的插入、删除、查找、遍历

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

typedef struct node
{
  int           id;
  struct node   *left;
  struct node   *right;
} node;
  
node *find(int id, node *root)
{ 
  if (root == NULL)
    return (NULL);
    
  if (id < root->id)
    return (find(id, root->left));
  else if (id > root->id)
    return (find(id, root->right));
  else
    return root;
}

node *findmin(node *root)
{ 
  if (root == NULL)
    return (NULL);
  
  if (root->left == NULL)
    return (root);
  else
    return (findmin(root->left));
}

node *insert(int id, node *root)
{
  if (root == NULL)
  { 
    root = malloc(sizeof(node));
    if (root == NULL)
    { 
      printf("out of memory\n");
      return (NULL);
    }
    root->id = id;
    root->left = root->right = NULL;
  }
  else if (id < root->id)
  { 
    root->left = insert(id, root->left);
  }
  else if(id > root->id)
  { 
    root->right = insert(id, root->right);
  }
  
  return (root);
}

node *delete(int id, node *root)
{ 
  node  *tmp;
  
  if (root == NULL)
    printf("not found");
  else if (id < root->id)
    root->left = delete(id, root->left);
  else if (id > root->id)
    root->right = delete(id, root->right);
  else
  { 
    if (root->left && root->right)
    { 
      tmp = findmin(root->right);
      root->id = tmp->id;
      root->right = delete(root->id, root->right);
    }
    else /* one or zero children */
    { 
      tmp = root;
      if (root->left == NULL)
        root = root->right;
      else if (root->right == NULL)
        root = root->left;
      free(tmp);
    }
  }

  return root;
}

oid retrieve(node *root) // preorder
{
  if (root)
  {
    printf("%d ", root->id);
    retrieve(root->left);
    retrieve(root->right);
  }
}

void inorder(node *root) // inorder
{
  if (root)
  {
    inorder(root->left);
    printf("%d ", root->id);
    inorder(root->right);
  }
}

void postorder(node *root) // postorder
{
  if (root)
  {
    postorder(root->left);
    postorder(root->right);
    printf("%d ", root->id);
  }
}

分别列了三种遍历方法：前序，中序（一般用于按顺序显示二叉搜索树的节点），后序。