Link: https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-tree/

Question

difficulty: mid
adj diff: 3

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 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.

Example 3:

Input: root = [1,2], p = 1, q = 2
Output: 1
Constraints:

	The number of nodes in the tree is in the range [2, 105].
	-109 <= Node.val <= 109
	All Node.val are unique.
	p != q
	p and q will exist in the tree.

Code

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public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    if (root == null)
        return null;
    if (root == p || root == q)
        return root;

    // depth-first search: find left and right
    TreeNode leftFound = lowestCommonAncestor(root.left, p, q);
    TreeNode rightFound = lowestCommonAncestor(root.right, p, q);

    if (leftFound == null) {
        return rightFound;
    } else if (rightFound == null) {
        return leftFound;
    } else {
        return root;
    }
}