将元素插入C中的BST

Question

I've got a structure type like the following:

typedef struct TreeNode{
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
} TreeNode;

To add an element to my

TreeNode* insert(TreeNode *root, int val){
TreeNode* a;
    a = root;
    int i;
    if(a==NULL){
        a -> val = val;
        a -> left = NULL;
        a -> right = NULL;
    }
    else if(a->val < val){
        return insert(root->left,val);
    }
    else if(a->val > val)
        return insert(root->right,val);
}

This gives no output when i evaluate it. What is my fault?

Answer 1

1. In the a == NULL case, you need to allocate memory for the node. Otherwise a -> val is illegal.
2. Also need to add a return to the code. You can return (a). Then when you call the function, you call it as root = insert(root, val)

The code is below.

TreeNode* insert(TreeNode *root, int val){
  TreeNode* a;
  a = root;
  int i;
  if(a==NULL){
    // Allocate memory here
    a = malloc(sizeof (root));
    if (a== NULL)
    {
       // Malloc error, You can exit the program or print a debug message here
    }

    a -> val = val;
    a -> left = NULL;
    a -> right = NULL;
  }
  else if(a->val < val){
    return insert(root->left,val);
  }
  else if(a->val > val)
    return insert(root->right,val);
  return a;
}