How to use sort() within a class

I am stuck with this problem.
See the code below

class BuildingHeightsEasy



struct node


int val;

int i;


bool myf(node p,node q)


return p.val<q.val;


int minimum(int M, vector > heights)


vector v;

sort(v.begin(),v.end(),myf); //THIS IS THROWING AN ERROR


if this is out of the class,then program is successfully compiled.Whats the problem??