Inserting in a Complete Binary Tree

I'm trying to insert into a complete binary tree for a school assignment. The structure of my tree is as such

typedef struct TreeNode {
void * data;
struct TreeNode * left;
struct TreeNode * right;
struct TreeNode * parent;
} TNode;

typedef struct CompleteBinaryTree {
TNode * root;
TNode * last;
int numelm;
} CBTree;

I have 4 ways of inserting depending the situation

• The tree is empty
• The tree as one node
• The last node of the CBT is his parent left child
• Finally I go up the tree as long as the current node is the right node of his parent, if the current node is not the root I will go to his brother and go down left, else I just go down left

I'm currently stuck on my third way and I don't know what is wrong with my code.

Could somebody push me in the right direction.

Thank you !

void CBTreeInsert(CBTree* tree, void* data) {
TNode * tmp = newTNode(data);
TNode * curr = tree->last;

if(tree->root == NULL){ //empty
    tree->root = tmp;
}
else if(tree->last == tree->root){ //one node
    tree->last->left = tmp;
}
else if(tree->last->parent->right == NULL){ //general
    tree->last->parent->right = tmp;
}
else if(tree->last->parent == tree->last->parent->right){ //degenarate
    while(curr->parent == curr->parent->right){
        curr = curr->parent;
    }
    if(curr != tree->root){
        curr = curr->parent->right;
        while(curr->left != NULL){
            curr = curr->left;
        }
        curr->left = tmp;
    }
    else{
        while(curr->left != NULL){
            curr = curr->left;
        }
        curr->left = tmp;
    }
}
else{
    fprintf(stderr,"Error\n");
}
tree->last = tmp;
tree->numelm++;
}