Trees are very important in data structures and algorithms topics, as an introduction of trees here i write some BST (Binary Search Trees) functions for find max depth, numbers of nodes, minimum value etc… I try to use less input variable and make the solutions recursive, if you find better solutions you can share it on your commands or post me for change the current one.

/*
 FINDS THE NUMBER OF NODES IN A TREE
*/
int size(struct node* node) {
  if (node==NULL) {
    return(0);
  } else {
    return(size(node->left) + 1 + size(node->right));
  }
} 

/*
 FINDS THE MAX DEPTH OF A TREE
*/
int maxDepth(struct node* node) {
  if (node==NULL) {
    return(0);
  }
  else {

    int lDepth = maxDepth(node->left);
    int rDepth = maxDepth(node->right);

    // use the larger one
    if (lDepth > rDepth) return(lDepth+1);
    else return(rDepth+1);
  }
} 

/*
 FINDS THE MIN VALUE DATA
*/
int minValue(struct node* node) {
  struct node* current = node;

  // loop for find the leftmost leaf
  while (current->left != NULL) {
    current = current->left;
  }

  return(current->data);
} 

/*
 CHANGES THE TREE LIKE MIRROR
*/
void mirror(struct node* node) {
  if (node==NULL) {
    return;
  }
  else {
    struct node* temp;

    // do the subtrees
    mirror(node->left);
    mirror(node->right);

    // swap the pointers
    temp = node->left;
    node->left = node->right;
    node->right = temp;
  }
} 

/*
 CHECKS IF THE TREES THE SAME
*/
int sameTree(struct node* a, struct node* b) {

  if (a==NULL && b==NULL) return(true);

  else if (a!=NULL && b!=NULL) {
    return(
      a->data == b->data &&
      sameTree(a->left, b->left) &&
      sameTree(a->right, b->right)
    );
  }

  else return(false);
}

/*
 RETURNS TRUE IF THE TREE IS BST AND min>=VALUES<=max */
int isBST(struct node* node) {
 return(isBST_(node, INT_MIN, INT_MAX));
 }
 int isBST_(struct node* node, int min, int max) {
 if (node==NULL) return(true);
 if (node->datadata>max) return(false);

  return
    isBST_(node->left, min, node->data) &&
    isBST_(node->right, node->data+1, max)
  );
}

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