Find the closest ancestor of two nodes in a tree. 

Discuss it!          


Here is some working C code... 


#include <stdio.h> 

typedef struct node 
{ 
  int value; 
  struct node *right; 
  struct node *left; 
}mynode; 

mynode *root; 

mynode *add_node(int value); 
void levelOrderTraversal(mynode *root); 
mynode *closestAncestor(mynode* root, mynode* p, mynode* q); 


int main(int argc, char* argv[]) 
{ 
  mynode *node_pointers[7], *temp; 
  root = NULL; 
   
  // Create the BST. 
  // Store the node pointers to use later... 
  node_pointers[0] = add_node(5); 
  node_pointers[1] = add_node(1); 
  node_pointers[2] = add_node(-20); 
  node_pointers[3] = add_node(100); 
  node_pointers[4] = add_node(23); 
  node_pointers[5] = add_node(67); 
  node_pointers[6] = add_node(13); 
   
     
  printf("\n\n\nLEVEL ORDER TRAVERSAL\n\n"); 
  levelOrderTraversal(root); 


  // Calculate the closest ancestors of a few nodes.. 

  temp = closestAncestor(root, node_pointers[5], node_pointers[6]); 
  printf("\n\nClosest ancestor of [%d] and [%d] is [%d]\n\n", 
         node_pointers[5]->value, 
         node_pointers[6]->value, 
         temp->value); 


  temp = closestAncestor(root, node_pointers[2], node_pointers[6]); 
  printf("\n\nClosest ancestor of [%d] and [%d] is [%d]\n\n", 
         node_pointers[2]->value, 
         node_pointers[6]->value, 
         temp->value); 
          
          
  temp = closestAncestor(root, node_pointers[4], node_pointers[5]); 
  printf("\n\nClosest ancestor of [%d] and [%d] is [%d]\n\n", 
         node_pointers[4]->value, 
         node_pointers[5]->value, 
         temp->value); 
          
          
  temp = closestAncestor(root, node_pointers[1], node_pointers[3]); 
  printf("\n\nClosest ancestor of [%d] and [%d] is [%d]\n\n", 
         node_pointers[1]->value, 
         node_pointers[3]->value, 
         temp->value);          


  temp = closestAncestor(root, node_pointers[2], node_pointers[6]); 
  printf("\n\nClosest ancestor of [%d] and [%d] is [%d]\n\n", 
         node_pointers[2]->value, 
         node_pointers[6]->value, 
         temp->value);   

} 


// Function to add a new node to the tree.. 
mynode *add_node(int value) 
{ 
   mynode *prev, *cur, *temp; 
    
   temp        = (mynode *) malloc(sizeof(mynode)); 
   temp->value = value; 
   temp->right = NULL; 
   temp->left  = NULL; 

   if(root==NULL) 
   { 
     printf("\nCreating the root..\n"); 
     root = temp; 
     return; 
   } 

   prev=NULL; 
   cur=root; 

   while(cur!=NULL) 
   { 
      prev=cur; 
      cur=(value<cur->value)?cur->left:cur->right; 
   } 

   if(value < prev->value) 
     prev->left=temp; 
   else 
     prev->right=temp; 
      
   return(temp);   
      
} 



// Level order traversal.. 
void levelOrderTraversal(mynode *root) 
{ 
  mynode *queue[100] = {(mynode *)0}; 
  int size = 0; 
  int queue_pointer = 0; 
   
  while(root) 
  { 
      printf("[%d] ", root->value); 

      if(root->left) 
      { 
        queue[size++] = root->left; 
      } 

      if(root->right) 
      { 
        queue[size++] = root->right; 
      } 
       
      root = queue[queue_pointer++];     
  } 
} 


// Function to find the closest ancestor... 
mynode *closestAncestor(mynode* root, mynode* p, mynode* q) 
{ 
   mynode *l, *r, *tmp; 

   if(root == NULL) 
   { 
      return(NULL); 
   } 
    
   if(root->left==p || root->right==p || root->left==q || root->right==q) 
   { 
     return(root); 
   } 
   else 
   { 
      l = closestAncestor(root->left, p, q); 
      r = closestAncestor(root->right, p, q); 
       
      if(l!=NULL && r!=NULL) 
      { 
        return(root); 
      }   
      else 
      { 
         tmp = (l!=NULL) ? l : r; 
         return(tmp); 
      } 
   } 
} 




Here is the tree for you to visualize... 


                        5 (node=0) 
         
       1 (node=1)                   100 (node=3) 

-20 (node=2)                 23 (node=4) 
                    
                   13 (node=5)   67 (node=6)   






Here is the output... 


LEVEL ORDER TRAVERSAL 

[5] [1] [100] [-20] [23] [13] [67] 


Closest ancestor of [67] and [13] is [23] 

Closest ancestor of [-20] and [13] is [5] 

Closest ancestor of [23] and [67] is [100] 

Closest ancestor of [1] and [100] is [5] 

Closest ancestor of [-20] and [13] is [5]