如果未找到值，C-搜索中的二進制搜索樹無法返回消息

Question

我嘗試使用遞歸製作二叉搜索樹。我只寫出了插入和搜索功能。但是，我的搜索功能有問題。我卡在第39行，如果我在樹中找不到值，它不會返回找不到該值的消息。請幫忙！

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


typedef struct node { 
    int key; 
    struct node* left; 
    struct node* right; 
}node; 

struct node* root= NULL; 

int contains(node* temp, int el){ 

     if (el==temp->key) { 
      return 1; 
     } 
     else if(el< temp->key) return contains(temp->left, el); 
     else return contains(temp->right, el); 
} 

void searchPrompt(void){ 
    int el=-1; 
    do{ 
     printf(" Search key or press -1 to return to menu: "); 
     scanf("%d", &el); 
     if(el>0){ 
      if (root==NULL) printf("\tError: tree is empty\n"); 
      else { 
       if(contains(root, el)) printf("\tKey %d is found\n",el); 
       else printf("\tKey %d is not found\n",el); 
      } 
     } 
     else if (el<-1||el==0) printf("\tError: key not positive\n"); 
    }while (el!=-1); 
    printf(" <Exit search method>\n\n"); 
} 
//for search 

void preOrder(node* temp){ 

    if (temp!=NULL){ 
     printf("%d ",temp->key); 
     preOrder(temp->left); 
     preOrder(temp->right); 
    } 
} 

//for insertion 
void insertNode(node* current, int value){   
     if(value< current->key){ 
      if (current->left == NULL) { 
       current->left=(node*) malloc(sizeof(node)); 
       current->left->key = value; 
       printf("\tSuccess! Value inserted: %d\n", current->left->key); 

      } 
      else { 
       insertNode(current->left, value); 
      } 
     } 
     else { 
      if (current->right == NULL) { 
       current->right=(node*) malloc(sizeof(node)); 
       current->right->key = value; 
       printf("\tSuccess! Value inserted: %d\n", current->right->key); 
      } 
      else { 
       insertNode(current->right, value); 
      } 
     } 
}//end insert 

void insert(int value){ 

    if(root==NULL){ //empty tree 
     root =(node*) malloc(sizeof(node)); 

     root->key= value; 
     printf("\tPrint root here: %d\n", value); 
     root->left= NULL; 
     root->right=NULL; 
     printf("\tSuccess! Value inserted: %d\n", root->key); 
    } 
    else { 
     insertNode(root, value); 
    }   
     printf("\tResult: "); 
     preOrder(root); 
     printf("\n"); 
} 

void insertPrompt(void){ 
    int value=-1; 
    do{ 
     printf(" Insert value or press -1 to return to menu: "); 
     scanf("%d", &value); 
     if(value>0) 
      insert(value); 
     else if (value<=0)printf("\tError: key not positive\n"); 
    }while (value!=-1); 
    printf(" <Exit insert method>\n\n"); 

} 

int menuPrompt(void){ 
    int choice=-1; 
    do{ 
     printf("Enter <1> Search <2> Insert <3> Delete <4> Print Tree <5> Quit: "); 
     scanf("%d", &choice); 
     if(choice>5 || choice<1) printf("Error: invalid input! \n\n"); 
    } while(choice>5 || choice<1); 
    return choice; 

} 

int main(int argc, char *argv[]){ 
    int choice=-1; 
    int value=-1; 


    while(choice!=5){ 

    choice=menuPrompt(); 

    switch(choice){ 
    case 1: 
     searchPrompt(); 
     break; 
    case 2: 
     insertPrompt(); 
     break; 
    case 3: 

     break; 
    case 4: 

     break;  
    case 5: 
     printf("<Exit program> \n"); 
     break; 
    }//end switch 

} 

    system("PAUSE"); 
    return 0; 
} 

Answer 1

int contains(node* temp, int el){ 
     if(temp==NULL) return 0; 
     if (el==temp->key) { 
      return 1; 
     } 
     else if(el< temp->key) return contains(temp->left, el); 
     else return contains(temp->right, el); 
} 

這解決了這兩個問題..saves你UB，也從錯誤行爲的情況下，elemnt沒有找到。

在BLUEPIXY提到了左右插入新節點到NULL。

爲什麼需要它，你可能會問..但事情是當你沒有找到 元素最終你會達到一些葉節點。然後，您將 左右移動，如果將其設置爲NULL，則會將contain 與NULL作爲參數，然後它會返回0，因爲您預期它會變爲 。

Answer 2

//for insertion 
void insertNode(node* current, int value){   
     if(value< current->key){ 
      if (current->left == NULL) { 
       current->left=(node*) malloc(sizeof(node)); 
       current->left->key = value; 
       current->left->left = NULL; 
       current->left->right = NULL; 
       printf("\tSuccess! Value inserted: %d\n", current->left->key); 

      } 
      else { 
       insertNode(current->left, value); 
      } 
     } 
     else { 
      if (current->right == NULL) { 
       current->right=(node*) malloc(sizeof(node)); 
       current->right->key = value; 
       current->right->left = NULL; 
       current->right->right = NULL; 
       printf("\tSuccess! Value inserted: %d\n", current->right->key); 
      } 
      else { 
       insertNode(current->right, value); 
      } 
     } 
}//end insert 

難道這就是我需要與我的新節點的左右呢？當傳遞一個NULL指針，當你搜索一個關鍵的不是樹，這將發生@BLUEPIXY @coderredoc