You can use 2 function one to build tree and other to query over the tree to find answer over the range A to B.

Code for buildtree :

void buildtree(int node,int b,int e){ if(b==e) M[node]=a[b]; else{ buildtree(2*node,b,(b+e)/2); buildtree(2*node+1,(b+e)/2+1,e); M[node]=min(M[2*node],M[2*node+1]); } }

Code for query from i to j:

int query(int node,int b,int e,int i,int j){ int p1, p2;

if (i>e || j<b) return inf;

  if (b>=i && e<=j) return M[node];

  p1 = query(2*node,b,(b+e)/2,i,j);
  p2 = query(2*node+1,(b+e)/2+1,e,i,j);

  return min(p1,p2);

}