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# Least Common Ancestor of any number of nodes in Binary Tree

• Difficulty Level : Easy
• Last Updated : 29 Jun, 2021

Given a binary tree (not a binary search tree) and any number of Key Nodes, the task is to find the least common ancestor of all the key Nodes.

Following is the definition of LCA from Wikipedia:
Let T be a rooted tree. The lowest common ancestor between two nodes n1 and n2 is defined as the lowest node in T that has both n1 and n2 as descendants (where we allow a node to be a descendant of itself).

The LCA of any number of nodes in T is the shared common ancestor of the nodes that is located farthest from the root. Example: In the figure above:

```LCA of nodes 12, 14, 15 is node 3
LCA of nodes 3, 14, 15 is node 3
LCA of nodes 6, 7, 15 is node 3
LCA of nodes 5, 13, 14, 15 is node 1
LCA of nodes 6, 12 is node 6```

Approach:
Following is the simple approach for Least Common Ancestor for any number of nodes.

• For every node calculate the matching number of nodes at that node and its sub-tree.
• If root is also a matching node.

matchingNodes = matchingNodes in left sub-tree + matchingNodes in right sub-tree + 1

• If root is not a matching node.

matchingNodes = matchingNodes in left sub-tree + matchingNodes in right-subtree

• If matching Nodes count at any node is equal to number of keys then add that node into the Ancestors list.
• The First node in the Ancestors List is the Least Common Ancestor of all the given keys.

Below is the implementation of above approach.

## C++14

 `// C++ implementation to find``// Ancestors of any number of nodes``#include ``using` `namespace` `std;` `// Tree Class``class` `TreeNode``{``    ``public``:``        ``int` `data;``        ``TreeNode *left, *right;``        ` `        ``TreeNode(``int` `value)``        ``{``            ``this``->data = value;``            ``this``->left = NULL;``            ``this``->right = NULL;``        ``}``};` `int` `getKeysCount(TreeNode *root, vector<``int``> &keyNodes,``                 ``int` `matchingNodes,``                 ``vector &ancestors)``{``    ` `    ``// Base Case. When root is Null``    ``if` `(root == NULL)``    ``{``        ``return` `0;``    ``}``    ` `    ``// Search for left and right subtree``    ``// for matching child Key Node.``    ``matchingNodes += getKeysCount(root->left, keyNodes,``                                  ``matchingNodes, ancestors) +``                     ``getKeysCount(root->right, keyNodes,``                                  ``matchingNodes, ancestors);``    ` `    ``// Condition to check if Root Node``    ``// is also in Key Node``    ``if` `(find(keyNodes.begin(),``             ``keyNodes.end(), root->data) != keyNodes.end())``    ``{``        ``matchingNodes++;``    ``}``    ` `    ``// Condition when matching Nodes is``    ``// equal to the Key Nodes found``    ``if` `(matchingNodes == keyNodes.size())``    ``{``        ``ancestors.push_back(root);``    ``}``    ` `    ``return` `matchingNodes;``}` `// Function to find Least Common``// Ancestors of N number of nodes``TreeNode *lcaOfNodes(TreeNode *root,``                     ``vector<``int``> &keyNodes)``{``    ` `    ``// Create a new list for``    ``// capturing all the ancestors``    ``// of the given nodes``    ``vector ancestors;``    ` `    ``// Initially there is No Matching Nodes``    ``int` `matchingNodes = 0;``    ``getKeysCount(root, keyNodes,``                 ``matchingNodes, ancestors);``    ` `    ``// First Node in the Ancestors list``    ``// is the Least Common Ancestor of``    ``// Given keyNodes``    ``return` `ancestors;``}` `// Driver Code``int` `main()``{``    ` `    ``// Creation of Tree``    ``TreeNode *root = ``new` `TreeNode(1);``    ` `    ``root->left = ``new` `TreeNode(2);``    ``root->right = ``new` `TreeNode(3);``    ``root->left->left = ``new` `TreeNode(4);``    ``root->left->right = ``new` `TreeNode(5);``    ``root->right->left = ``new` `TreeNode(6);``    ``root->right->right = ``new` `TreeNode(7);``    ``root->left->left->left = ``new` `TreeNode(8);``    ``root->left->left->right = ``new` `TreeNode(9);``    ``root->left->right->left = ``new` `TreeNode(10);``    ``root->left->right->right = ``new` `TreeNode(11);``    ``root->right->left->left = ``new` `TreeNode(12);``    ``root->right->left->right = ``new` `TreeNode(13);``    ``root->right->right->left = ``new` `TreeNode(14);``    ``root->right->right->right = ``new` `TreeNode(15);``    ` `    ``// Key Nodes for LCA``    ``vector<``int``> keyNodes;``    ``keyNodes.push_back(12);``    ``keyNodes.push_back(14);``    ``keyNodes.push_back(15);``    ` `    ``cout << lcaOfNodes(root, keyNodes)->data``         ``<< endl;``    ` `    ``return` `0;``}` `// This code is contributed by sanjeev2552`

## Java

 `// Java implementation to find``// Ancestors of any number of nodes``import` `java.util.ArrayList;` `// Tree Class``class` `TreeNode {``    ``int` `data;``    ``TreeNode left;``    ``TreeNode right;` `    ``public` `TreeNode(``int` `value)``    ``{``        ``this``.data = value;``        ``left = right = ``null``;``    ``}``}` `public` `class` `LCAofAnyNumberOfNodes {``    ` `    ``// Function to find Least Common``    ``// Ancestors of N number of nodes``    ``public` `static` `TreeNode lcaOfNodes(``        ``TreeNode root,``        ``ArrayList keyNodes)``    ``{``        ``// Create a new list for``        ``// capturing all the ancestors``        ``// of the given nodes``        ``ArrayList ancestors =``                    ``new` `ArrayList();``        ` `        ``// Initially there is No Matching Nodes``        ``int` `matchingNodes = ``0``;``        ``getKeysCount(root, keyNodes,``                 ``matchingNodes, ancestors);` `        ``// First Node in the Ancestors list``        ``// is the Least Common Ancestor of``        ``// Given keyNodes``        ``return` `ancestors.get(``0``);``    ``}` `    ``private` `static` `int` `getKeysCount(``        ``TreeNode root, ArrayList keyNodes,``        ``int` `matchingNodes,``        ``ArrayList ancestors)``    ``{``        ``// Base Case. When root is Null``        ``if` `(root == ``null``)``            ``return` `0``;` `        ``// Search for left and right subtree``        ``// for matching child Key Node.``        ``matchingNodes += getKeysCount(root.left,``                ``keyNodes, matchingNodes, ancestors)``            ``+ getKeysCount(root.right,``                ``keyNodes, matchingNodes, ancestors);``        ` `        ``// Condition to check if Root Node ``        ``// is also in Key Node``        ``if` `(keyNodes.contains(root.data)){``            ``matchingNodes++;``        ``}` `        ``// Condition when matching Nodes is``        ``// equal to the Key Nodes found``        ``if` `(matchingNodes == keyNodes.size())``            ``ancestors.add(root);``        ``return` `matchingNodes;``    ``}``    ` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ` `        ``// Creation of Tree``        ``TreeNode root = ``new` `TreeNode(``1``);` `        ``root.left = ``new` `TreeNode(``2``);``        ``root.right = ``new` `TreeNode(``3``);``        ``root.left.left = ``new` `TreeNode(``4``);``        ``root.left.right =``                        ``new` `TreeNode(``5``);``        ``root.right.left =``                        ``new` `TreeNode(``6``);``        ``root.right.right =``                        ``new` `TreeNode(``7``);``        ``root.left.left.left =``                        ``new` `TreeNode(``8``);``        ``root.left.left.right =``                        ``new` `TreeNode(``9``);``        ``root.left.right.left =``                        ``new` `TreeNode(``10``);``        ``root.left.right.right =``                        ``new` `TreeNode(``11``);``        ``root.right.left.left =``                        ``new` `TreeNode(``12``);``        ``root.right.left.right =``                        ``new` `TreeNode(``13``);``        ``root.right.right.left =``                        ``new` `TreeNode(``14``);``        ``root.right.right.right =``                        ``new` `TreeNode(``15``);``        ` `        ``// Key Nodes for LCA``        ``ArrayList keyNodes =``                ``new` `ArrayList();``        ``keyNodes.add(``12``);``        ``keyNodes.add(``14``);``        ``keyNodes.add(``15``);``        ``System.out.println(``            ``lcaOfNodes(root, keyNodes).data``        ``);``    ``}``}`

## C#

 `// C# implementation to find``// Ancestors of any number of nodes``using` `System;``using` `System.Collections.Generic;` `// Tree Class``class` `TreeNode {``    ``public` `int` `data;``    ``public` `TreeNode left;``    ``public` `TreeNode right;`` ` `    ``public` `TreeNode(``int` `value)``    ``{``        ``this``.data = value;``        ``left = right = ``null``;``    ``}``}`` ` `public` `class` `LCAofAnyNumberOfNodes {``     ` `    ``// Function to find Least Common``    ``// Ancestors of N number of nodes``    ``static` `TreeNode lcaOfNodes(``        ``TreeNode root,``        ``List<``int``> keyNodes)``    ``{``        ``// Create a new list for``        ``// capturing all the ancestors``        ``// of the given nodes``        ``List ancestors =``                    ``new` `List();``         ` `        ``// Initially there is No Matching Nodes``        ``int` `matchingNodes = 0;``        ``getKeysCount(root, keyNodes,``                 ``matchingNodes, ancestors);`` ` `        ``// First Node in the Ancestors list``        ``// is the Least Common Ancestor of``        ``// Given keyNodes``        ``return` `ancestors;``    ``}`` ` `    ``private` `static` `int` `getKeysCount(``        ``TreeNode root, List<``int``> keyNodes,``        ``int` `matchingNodes,``        ``List ancestors)``    ``{``        ``// Base Case. When root is Null``        ``if` `(root == ``null``)``            ``return` `0;`` ` `        ``// Search for left and right subtree``        ``// for matching child Key Node.``        ``matchingNodes += getKeysCount(root.left,``                ``keyNodes, matchingNodes, ancestors)``            ``+ getKeysCount(root.right,``                ``keyNodes, matchingNodes, ancestors);``         ` `        ``// Condition to check if Root Node ``        ``// is also in Key Node``        ``if` `(keyNodes.Contains(root.data)){``            ``matchingNodes++;``        ``}`` ` `        ``// Condition when matching Nodes is``        ``// equal to the Key Nodes found``        ``if` `(matchingNodes == keyNodes.Count)``            ``ancestors.Add(root);``        ``return` `matchingNodes;``    ``}``     ` `    ``// Driver Code``    ``public` `static` `void` `Main(String[] args)``    ``{``         ` `        ``// Creation of Tree``        ``TreeNode root = ``new` `TreeNode(1);`` ` `        ``root.left = ``new` `TreeNode(2);``        ``root.right = ``new` `TreeNode(3);``        ``root.left.left = ``new` `TreeNode(4);``        ``root.left.right =``                        ``new` `TreeNode(5);``        ``root.right.left =``                        ``new` `TreeNode(6);``        ``root.right.right =``                        ``new` `TreeNode(7);``        ``root.left.left.left =``                        ``new` `TreeNode(8);``        ``root.left.left.right =``                        ``new` `TreeNode(9);``        ``root.left.right.left =``                        ``new` `TreeNode(10);``        ``root.left.right.right =``                        ``new` `TreeNode(11);``        ``root.right.left.left =``                        ``new` `TreeNode(12);``        ``root.right.left.right =``                        ``new` `TreeNode(13);``        ``root.right.right.left =``                        ``new` `TreeNode(14);``        ``root.right.right.right =``                        ``new` `TreeNode(15);``         ` `        ``// Key Nodes for LCA``        ``List<``int``> keyNodes = ``new` `List<``int``>();``        ``keyNodes.Add(12);``        ``keyNodes.Add(14);``        ``keyNodes.Add(15);``        ``Console.WriteLine(``            ``lcaOfNodes(root, keyNodes).data``        ``);``    ``}``}` `// This code is contributed by PrinciRaj1992`

## Javascript

 ``
Output:
`3`

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