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).”

Given the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]

Cases:

1. currentNode is null or p or q, return currentNode;

2. leftChild return a tree_node and rightChild return a tree_node, return currentNode

3. leftChild return a tree_node m and rightChild return null:

return m

4. rightChild return a tree_node m and leftChild return null:

return m

/**
 * 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 &&right!=null) return root;
        else if(left!=null&&right==null){
            return left;
        }
        else{
            return right;
        }
    }
}      

leetcode 236-Lowest Common Ancestor of a Binary Tree(medium)