Same Tree

class Solution {
  public boolean isSameTree(TreeNode p, TreeNode q) {
    // p and q are both null
    if (p == null && q == null) return true;
    // one of p and q is null
    if (q == null || p == null) return false;
    if (p.val != q.val) return false;
    return isSameTree(p.right, q.right) &&
            isSameTree(p.left, q.left);
  }
}