lowest common ancestor in bt (dsa 450)

450 DSA/Binary Trees/Lowest common acestor in binary tree/Lowest common ancestor in binary tree.cpp

@@ -0,0 +1,110 @@
+/*
+Link to the question:
+https://practice.geeksforgeeks.org/problems/lowest-common-ancestor-in-a-binary-tree/1
+
+Given a Binary Tree with all unique values and two nodes value n1 and n2. 
+The task is to find the lowest common ancestor of the given two nodes. 
+We may assume that either both n1 and n2 are present in the tree or none of them is present. 
+
+Input:
+n1 = 3 , n2 = 4
+         5
+        /
+       2
+     /  \
+    3    4
+Output: 2
+
+*/
+class Solution
+{
+    public:
+    //Function to return the lowest common ancestor in a Binary Tree.
+
+    // global variable ans to store the answer.
+    Node *ans=NULL;
+
+
+    bool solve(Node *root,int n1,int n2)
+    {
+        // if root is null return false.
+        if(!root)
+        {
+            return false;
+        }
+        // call the solve function for the left child
+        // returns true if it finds any on the node in the left subtree
+        bool l=solve(root->left,n1,n2);
+
+        // call the solve function for the right child
+        bool r=solve(root->right,n1,n2);
+
+        // if root is one of the nodes then
+        if(root->data==n1 || root->data==n2)
+        {
+            // check if we find the other node in its left or right subtree
+            if(l || r)
+            {
+                // if found then lca becomes the root.
+                // change ans from null to root.
+                ans=root;
+
+                return true;
+            }
+            return true;
+        }
+        // if both the nodes are found in the left and right subtree then   
+        if(l && r)
+        {
+            // if ans is null then only the root will be lca else root will be the common ancestor but not lowest common ancestor.
+            if(!ans)
+            {
+            ans=root;
+            }
+
+            return true;
+        }
+        // if node found in left subtree return true
+        else if(l)
+        {
+            return true;
+
+        }
+        // if node found in right subtree return true
+        else if(r)
+        {
+            return true;
+        }
+        // if no node found in left and right return false
+        return false;
+    }
+
+
+
+    Node* lca(Node* root ,int n1 ,int n2 )
+    {
+        // if the root itself is null then return NULL
+        if(!root)
+        {
+            return NULL;
+        }
+
+        // if any one of the given two nodes is the root then LCA of that two nodes will obviously be the root node.
+        if(n1==root->data || n2==root->data)
+        {
+            return root;
+        }     
+
+        // call the solve function passing the root and the nodes whose lca has to be calculated.
+        bool a1=solve(root,n1,n2);
+
+        // if both the nodes are not present then a1 will be false hence return NULL
+        if(!a1)
+        {
+            return NULL;
+        }
+        // else return the ans 
+        return ans;
+
+    }
+};

450 DSA/Binary Trees/README.md

@@ -3,4 +3,4 @@
 #### Questions in Binary Trees:
 | # | Question |
 |---|----------|
-| 1 | ...      |
+| 1 | Lowest common ancestor in binary tree   |