Design an algorithm and write code to find the first common ancestor of two nodes in a binary tree. Avoid storing additional nodes in a data structure. NOTE: This is not necessarily a binary search tree.

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

    3
   / \
  5   1
 / \ / \
6  2 0  8
  / \
 7   4

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Input: 3
Explanation: The first common ancestor of node 5 and node 1 is node 3.

Example 2:

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

Notes:

• All node values are pairwise distinct.
• p, q are different node and both can be found in the given tree.