Pair with minimum absolute difference | BST

Given a binary search tree of size N > 1, the task is to find the minimum absolute difference between any two nodes.


        /   \ 
       3     7 
      / \   / \ 
     2   4 6   8
Output: 1
Difference between all the consecutive nodes if sorted is 1.
Thus, the answer is 1.

Output: 5

Approach: We know that in-order traversal of a Binary Search Tree traverses it in sorted order. So, for every node, we will find its difference from the previous node in the in-order traversal of the tree. If this difference is smaller than the previous minimum difference, we will update the previous minimum difference. Following are the steps to follow:

  1. Create a variable ‘prev’ to store the pointer to the previous node in in-order traversal.
  2. Create a variable ‘ans’ to store the minimum difference.
  3. For every node in the in-order traversal, compare its absolute difference with the previous node and update the minimum absolute difference found so far.

Below is the implementation of the above approach:

// C++ implementation of the approach
using namespace std;

// Node of the binary tree
struct node {
int data;
node* left;
node* right;
node(int data)
this->data = data;
left = NULL;
right = NULL;

// Function for in-order traversal of the tree
void inorder(node* curr, node*& prev, int& ans)

// Base-case
if (curr == NULL)

// Calling in-order on the left sub-tree
inorder(curr->left, prev, ans);

if (prev != NULL)
ans = min(curr->data – prev->data, ans);
prev = curr;

// Calling in-order on the right sub-tree
inorder(curr->right, prev, ans);

// Function to return the minimum
// difference between any two nodes
// of the given binary search tree
int minDiff(node* root)

// Pointer to previous node in the
// in-order traversal of the BST
node* prev = NULL;

// To store the final ans
int ans = INT_MAX;

// Call in-order for the BST
inorder(root, prev, ans);

// Returning the final answer
return ans;

// Driver code
int main()
node* root = new node(5);
root->left = new node(3);
root->right = new node(7);
root->left->left = new node(2);
root->left->right = new node(4);
root->right->left = new node(6);
root->right->right = new node(8);

cout << minDiff(root); return 0; } Another Approach with O(1) Space Complexity:

# Python 3 implementation of the approach

# Node of the binary tree
import math
class Node:

# Constructor to create a new node
def __init__(self, data): = data
self.left = None
self.right = None
#Set the target to infinity
#Function to find the minimum absolute difference
def absolute_diff(root,target):
if root is None:
return target

if root.left is not None:
if root.right is not None:
#Find the minimum in the left subtree
#Find the minimum in the right subtree
return min(p,q)

// Driver code
root.left = Node(3)
root.right = Node(7)
root.left.left = Node(2)
root.left.right = Node(4)
root.right.left = Node(6)
root.right.right = Node(8)



Time complexity: O(N)
Additional Space: O(1)

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