Do iterative traversal of second tree and find first lead node in it as " temp2"
Node * temp2 = s2 . top ( ) ; s2 . pop ( ) ; while ( temp2 && ! isLeaf ( temp2 ) ) { s2 . push ( temp2 -> right ) ; s2 . push ( temp2 -> left ) ; temp2 = s2 . top ( ) ; s2 . pop ( ) ; }