236. Lowest Common Ancestor of a Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null) {
            return null;
        }

        // it means one of them is the ancestor of the another
        if (root == p || root == q) {
            return root;
        }

        // if the root is not LCA, try to find it in the left tree
        TreeNode leftLCA = lowestCommonAncestor(root.left, p, q);

        // if the root is not LCA, try to find it in the right tree
        TreeNode rightLCA = lowestCommonAncestor(root.right, p, q);

        if (leftLCA == null) {
            return rightLCA;
        } else if (rightLCA == null) {
            return leftLCA;
        } else {
            return root;
        }
    }
}