递归搜索并插入二叉树

Question

first of all happy new year. I was trying to fix a piece of code I've been stumped on for hours. (Note: I am not a solid coder.)

What I am trying to do is to write a function "searchInsert", which will take in a binary tree and some integer i. It will then try to find the integer i in the tree. If it's not there it is inserted into the tree.

Other information: If we do in fact find the integer in the tree, return a pointer pointing to the node of to it. If we do not find it as we said before, insert it in BUT return a pointer pointing to the root of the tree.

I also must do this recursively.

Now I have tested it using an arbitrary tree along with i = 98, as follows:

Before what it looks like.

     4
    / \
   2   6
  / \ / \
 1  3 5  7

After, what it should look like:

     4
    / \
   2   6
  / \ / \
 1  3 5  7
          \
          98

But my code doesn't seem to be working.

treelink searchInsert(treelink t, TreeItem i){

    treelink keyNode = NULL;
    if (t == NULL) {
        t = insertTreeNode(t, i);  
    } else if(i < t->item){
        keyNode = searchInsert(t->left,i); 
    } else if(i > t->item){
        keyNode = searchInsert(t->right,i);   
    } else {
        keyNode = t;
        return keyNode;
    }    

    return t;
 }

Other important notes: treelink is a pointer to a binary tree. Assume insertTreeNode works as it was a function given to us.

Any help would be appreciated thanks.

Answer 1

Among other problems, you have lost all context when you realize that you have not found the item that you are looking for:

if ( t == NULL ) {
  t = insertTreeNode(t, i) ;
}

So you are always calling insertTreeNode with NULL as the first argument.

While recursion is a great way to step through a tree, you might instead want to create a pointer and iterate through the tree, so that you have the original t around when you decide to call insert.

{
  treelink ptr= t ;
  while ( ptr )
  {
    if ( ptr-> item == i ) return ptr ;
    ptr= ( ptr-> item > i ) ? ptr-> left : ptr-> right ;
  }

  return insertTreeNode( t, i ) ;
}

Answer 2

A new node is created, but it is not linked to the tree. You never change your left and right pointers.

what you need is to update the link after recursive call, eg:

else if (i < t->item) {
    t->left = searchInsert(t->left, i);
} ...

But then of course you cannot simply return a pointer to a found item if it's found, otherwise it would break the tree. That's because the statement of your task is NOT recursive: you have to return either root or an existing (inner) node. So you might want to write a recursive function which eg always returns a pointer to the root, but also returns a pointer to a found item (through an additional treelink* argument).

Or maybe it would be simpler to split the function into two: search which returns a pointer to an existing node, and insert which returns a pointer to the root. Both of them would be recursive and quite simple.