Lowest Common Ancestor in General Binary Tree

Given the root of a binary tree and two distinct nodes `nodeA` and `nodeB` (guaranteed to exist in the tree), find the **lowest common ancestor (LCA)** of these two nodes.

The LCA is defined as the deepest node that has both `nodeA` and `nodeB` as descendants (a node is considered a descendant of itself).

**Example 1:**
```
10
/ \
6 15
/ \ / \
3 8 12 20
/
7
```
Input: root = [10,6,15,3,8,12,20,null,null,7], nodeA = 6, nodeB = 7
Output: 6
Explanation: Node 7 is a descendant of node 6, so the LCA is node 6 itself.

**Example 2:**
```
(same tree)
```
Input: root = [10,6,15,3,8,12,20,null,null,7], nodeA = 3, nodeB = 20
Output: 10
Explanation: Node 3 is in the left subtree and node 20 is in the right subtree, so the LCA is the root 10.

**Note:** You may assume that node values are unique within the tree, and that both target values are guaranteed to exist.