c - AVL tree insert function -

avltree insert( elementtype x, avltree t) {   1   if( t == null)   2  {   3     /* create , return one-onde tree */   4      t = malloc( sizeof( struct avlnode ) );   5      if( t == null )   6          fatalerror( "out of space!!!");   7      else   8     {   9         t->element = x; t->height = 0;  10         t->left = t->right = null;  11       }  12   }  13   else  14   if( x < t->element )  15   {  16       t->left = insert(x, t->left );  17       if( height( t->left ) - height( t->right ) == 2 )  18           if( x < t->left->element )  19               t = singlerotatewithleft( t );  20           else  21               t = doublerotatewithleft( t );  22    }  23    else  24    if( x > t->element )  25   {  26        t->right = insert( x, t->right );  27        if( height( t->right ) - height( t->left ) == 2 )  28            if( x > t->right->element )  29                t = singlerotatewithright( t );  30            else  31                t = doublerotatewithright( t );  32    }  33    /* else x in tree already; we'll nothing */  34    t->height = max( height( t->left ), height( t->right ) ) + 1;  35    return t; } 

why x isn't equal t->left->element, , how can x compare t->left->element x should assign t->left-element(when t->left = null)

after line 18, x has been inserted t->left, may not (necessarily) @ top of tree.