Lowest Common Ancestor of a Binary Tree

Problem statement

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:

Input:

root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1

Expected output:

3

Explanation: The LCA of nodes 5 and 1 is 3.

If you would like to solve the problem on LeetCode, here is the link to the problem: LeetCode problem link

Golang Solution

Recurse on the left and right subtrees. If the current node is p or q, return it. If both sides return a non-nil node, the current root is the LCA. Otherwise bubble up whichever side found a match.

Time: O(n)
Space: O(h) recursion stack (h = tree height)

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func lowestCommonAncestor(root, p, q *TreeNode) *TreeNode {
	if root == nil {
		return nil
	}

	if root.Val == p.Val {
		return p
	}

	if root.Val == q.Val {
		return q
	}

	l := lowestCommonAncestor(root.Left, p, q)
	r := lowestCommonAncestor(root.Right, p, q)

	if l != nil && r != nil {
		return root
	}

	if l != nil {
		return l
	} else {
		return r
	}
}