Lowest Common Ancestor of a Binary Tree

Problem statement

Given a binary tree and two nodes p and q, return their lowest common ancestor: the deepest node that has both as descendants (a node may be an ancestor of itself).

Example:

Input:

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

Expected output:

3

Why: both 5 and 1 sit under 3, and nothing deeper covers both.

Practice on LeetCode: Lowest Common Ancestor of a Binary Tree

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
	}
}