View Compilation Error

0_0_19437587_22888.cpp: In function 'node* merge(node*, node*)':
0_0_19437587_22888.cpp:51:57: error: cannot convert 'node*' to 'node**' for argument '1' to 'void swap(node**, node**)'
     if(a->l==NULL || a->r->dis>a->l->dis) swap(a->l,a->r);  
                                                         ^