code: private void swap(int n, int m) {
  		int eM = elements[m];
  		int eN = elements[n];
  		elements[m] = eN;
  		elements[n] = eM;
  		positions[eM] = n;
  		positions[eN] = m;
  	}
docs: swap data in node n with data in node m
  
  @param n index of the node containing a data
  @param m index of the node containing a data