236. Lowest Common Ancestor of a Binary Tree

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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]


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.

Note:
  • All of the nodes' values will be unique.
  • p and q are different and both values will exist in the binary tree.
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Intuition
DFS - need to check all nodes

At each node check

1. If current node = p || q, return current node
2. Recurse left, right
3. If both left, and right are not null, we found 1st node above them, return current node - LCA
4. If one of them is not null, return the not null node
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Time - O(n)
Space - O(1) - ignore function call stack - O(n) - consider function call stack and tree is skewed to one side
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Related problems
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