Solution2.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private TreeNode u;
    private TreeNode ans;
    private int d;

    public TreeNode findNearestRightNode(TreeNode root, TreeNode u) {
        this.u = u;
        dfs(root, 1);
        return ans;
    }

    private void dfs(TreeNode root, int i) {
        if (root == null || ans != null) {
            return;
        }
        if (d == i) {
            ans = root;
            return;
        }
        if (root == u) {
            d = i;
            return;
        }
        dfs(root.left, i + 1);
        dfs(root.right, i + 1);
    }
}