Given a Binary Search Tree, Find the distance between 2 nodes

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    Write a function that given a BST, it will return the distance between 2 nodes.

    For example, given this tree

            / \
           B   C
          / \   \
         D   E   F
        /         \
       G           H

    The distance between G and F is 3: [G -> D -> B -> E]

    The distance between E and H is 6: [G -> D -> B -> A -> C -> F -> H]

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    The examples seem a bit wrong. The path from G to F should be: G->D->B->A->C->F, and the path from E to H should be E->B->A->C->F->H.

    The obvious solution uses bottom-up approach, which takes worst case O(n) time. A similar problem on Leetcode is a question called "maximum path sum" I think.

    Pseudocode (without much verification):

    def findDistance(TreeNode root, TreeNode p, TreeNode q):
    if root == null or p == q:
    	return 0
    distance_left = findDistance(root.left, p, q)
    distance_right = findDistance(root.right, p, q)
    if (distance_left > 0 and distance_right > 0):
    	return distance_left + distance_right
    if (distance_left > 0 and root is p or q):
    	return distance_left
    if (distance_right > 0 and root is p or q):
    	return distance_right
    if (distance_left == 0 and distance_right == 0):
    	return 0 if root is not p and not q
    	return 1 if root is p or q
    	return max(distance_left, distance_right) + 1

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    A couple of flaws in your algorithm: 1) The last line wrongly assumes the current node i.e. root in method argument is always on the resulting path, hence your algorithm will yield 3 if the input is D and E in the above tree. 2) Your algorithm cannot handle the case where one of the target node is not in the tree. The solution: you need to record the number of matches found as well.

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    You have to modify the solution for Least common ancestor.

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    Last line returns the current tentative distance if only one of the nodes is found. Since I'm using a bottom-up approach, if only one node is found in a subtree and the other is not found, in the end the path must go through the current node (root). I don't see a problem returning 3 if the input is D and E (D->B->E). My solution does not handle the case when a node or both nodes are not in the tree.

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    C# solution: to first find the LCA, and then calculate the level of root->LCA/root->node1/root->node2, then distance = root->node1 + root->node2 - 2 * root->LCA;

        class MyTreeNode
            public int Data { get; set; }
            public MyTreeNode Left { get; set; }
            public MyTreeNode Right { get; set; }
            public MyTreeNode(int data)
                this.Data = data;
        class QTwoNodeDis
            public static int Distance(MyTreeNode root, MyTreeNode node1, MyTreeNode node2)
                var node = FindLCA(root, node1, node2);
                int distLCA = FindLevel(root, node);
                int dist1 = FindLevel(root, node1);
                int dist2 = FindLevel(root, node2);
                return dist1 + dist2 - 2 * distLCA;
            private static MyTreeNode FindLCA(MyTreeNode root, MyTreeNode node1, MyTreeNode node2)
                if (root == null) return null;
                if (root.Data == node1.Data || root.Data== node2.Data)
                    return root;
                MyTreeNode left_lca = FindLCA(root.Left, node1, node2);
                MyTreeNode right_lca = FindLCA(root.Right, node1, node2);
                if (left_lca != null && right_lca != null)
                    return root;
                return left_lca != null ? left_lca : right_lca;
            private static int FindLevel(MyTreeNode root, MyTreeNode node)
                if (root == null)
                    return -1;
                if(root.Data == node.Data)
                    return 0;
                int level = FindLevel(root.Left, node);
                if (level == -1)
                    level = FindLevel(root.Right, node);
                if(level != -1)
                    return level + 1;
                return -1;

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    I don't think the example is a BST...

    If the problem input is a BST, the solution above is OK, but if it is just a Binary Tree, what should we do?

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    I think this will work if its BST. did you think of any case where this code will fail ?

    import java.util.*;
    public class DistanceBetweenBSTNode{
    public int distance(TreeNode root, TreeNode node1,TreeNode node2){
    	if(root == null || node1 == node2)
    		return 0;
    	if(root.val >= node1.val && root.val <= node2.val)
    		return distance(root,node1) + distance(root,node2);
    	else if(root.val < node1.val && root.val < node2.val)
    		return distance(root.right,node1,node2);
    		return distance(root.left,node1,node2);
    public int distance(TreeNode root, TreeNode node1){
    	if(root == node1)
    		return 0;
    	if(root.val < node1.val){
    		return 1+ distance(root.right,node1);
    		return 1+ distance(root.left,node1);
    public static void main(String[] args){
    	DistanceBetweenBSTNode dbb = new DistanceBetweenBSTNode();
    	TreeNode root = new TreeNode(5);
    	root.left = new TreeNode(3);
    	root.right =  new TreeNode(10);
    	root.right.right = new TreeNode(11);
    	root.right.left = new TreeNode(8);

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