In this post, we are going to solve the 236. Lowest Common Ancestor of a Binary Tree problem of Leetcode. This problem 236. Lowest Common Ancestor of a Binary Tree is a Leetcode medium level problem. Let’s see the code, 236. Lowest Common Ancestor of a Binary Tree – Leetcode Solution.

Problem

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Example 1 :

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.

Example 2 :

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

Example 3 :

Input: root = [1,2], p = 1, q = 2
Output: 1

Constraints

• The number of nodes in the tree is in the range [2, 105].
• -109 <= Node.val <= 109
• All Node.val are unique.
• p != q
• p and q will exist in the tree.

Now, let’s see the code of 236. Lowest Common Ancestor of a Binary Tree – Leetcode Solution.

Lowest Common Ancestor of a Binary Tree – Leetcode Solution

236. Lowest Common Ancestor of a Binary Tree – Solution in Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root == null || root == p || root == q) return root;
        
        TreeNode left = lowestCommonAncestor(root.left,p,q);
        TreeNode right = lowestCommonAncestor(root.right,p,q);
        
        if(left == null){
            return right;
        } else if(right == null){
            return left;
        }else{
            return root;
        }
    }
}

236. Lowest Common Ancestor of a Binary Tree – Solution in C++

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (!root || root == p || root == q) return root;
        TreeNode* left = lowestCommonAncestor(root->left, p, q);
        TreeNode* right = lowestCommonAncestor(root->right, p, q);
        return !left ? right : !right ? left : root;
    }
};

236. Lowest Common Ancestor of a Binary Tree – Solution in Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def lowestCommonAncestor(self, root, p, q):
        if root in (None, p, q): return root
        left, right = (self.lowestCommonAncestor(kid, p, q)
                       for kid in (root.left, root.right))
        return root if left and right else left or right
        

Note: This problem 236. Lowest Common Ancestor of a Binary Tree is generated by Leetcode but the solution is provided by CodingBroz. This tutorial is only for Educational and Learning purpose.