CousinBT.java

class Solution {
    public boolean isCousins(TreeNode root, int x, int y) {
    TreeNode xx = findNode(root, x);
    TreeNode yy = findNode(root, y);

    return (
      (level(root, xx, 0) == level(root, yy, 0)) && (!isSibling(root, xx, yy))
    );
  }

  TreeNode findNode(TreeNode node, int x) {
    if (node == null) {
      return null;
    }
    if (node.val == x) {
      return node;
    }
    TreeNode n = findNode(node.left, x);
    if (n != null) {
      return n;
    }
    return findNode(node.right, x);
  }

  boolean isSibling (TreeNode node, TreeNode x, TreeNode y) {
    if (node == null) {
      return false;
    }

    return (
      (node.left == x && node.right == y) || (node.left == y && node.right == x)
      || isSibling(node.left, x, y) || isSibling(node.right, x, y)
    );
  }

  int level (TreeNode node, TreeNode x, int lev) {
    if(node == null) {
      return 0;
    }

    if(node == x) {
      return lev;
    }

    int l = level(node.left, x, lev+1);
    if (l != 0) {
      return l;
    }
    return level(node.right, x, lev+1);
  }
}