iteratively insert into a binary search tree.Debug C++ code - c++

Question

Here is a c++ function to create a BST tree from an array of integers?
It's simple.
Take first element ,make root.
Take next array element and insert it into the tree.
Why is the loop starting from i=2 and not i=1??
node* buildtree(int a[], int len)
{
node* root=new node(a[0]);
node* temp=root;
for(int i=1;i<len;i++)
{
while(!(root->left==NULL && root->right==NULL))
{
cout<<"here"<<i<<" "<<a[i]<<" " << root->val<<"\n";
if(root->val>a[i])
root=root->left;
else
root=root->right;
}
node* currnode=new node(a[i]);
if(root->val>a[i])
root->left=currnode;
else
root->right=currnode;
if(root==NULL)
cout<<"error...never.here";
root=temp;
}
return root;
}
Thanks a lot for explaining it.I tried it another way but it only finds the root.What's the problem in it?
node* buildtree(int a[],int len)
{ node* root=new node(a[0]);
node* curr;
for(int i=1;i<len;i++)
{ curr=root;
while(curr!=NULL)
{
if(curr->val>a[i])
curr=curr->left;
else
curr=curr->right;
}
curr=new node(a[i]);
}
return root;
}

Answer 1

Because in the first iteration of the loop the while condition is not true because the root node has no child nodes.
while(!(root->left==NULL && root->right==NULL)
for i=1 the left and the right node are NULL and the left node is populated at the end of the first iteration.

Answer 2

When trying to find the point of insertion,
while(!(root->left==NULL && root->right==NULL))
{
cout<<"here"<<i<<" "<<a[i]<<" " << root->val<<"\n";
if(root->val>a[i])
root=root->left;
else
root=root->right;
}
you only stop if both children are NULL, so at some point or other, you will set root to NULL. Consider the array begins with [5, 3, 6, ... ]. You start with
NULL <- node(5) -> NULL
node(3) <- node(5) ->NULL
and then try to insert the 3. Since not both children are NULL, the while loop runs
if (5 > 7) // false
root = root->left;
else
root = root->right; // now root == NULL, oops
and the controlling condition is checked anew
while(!(NULL->left == NULL && NULL->right == NULL))
segfault likely here, undefined behaviour invoked.
You should do something like
while(true) {
if (root->val > a[i]) {
if (root->left == NULL) {
root->left = new node(a[i]);
break;
} else {
root = root->left;
}
} else {
if (root->right == NULL) {
root->right = new node(a[i]);
break;
} else {
root = root->right;
}
}
}