ZigZag2[size_] := Module[{data, i, j, elem},
 data = ConstantArray[0, {size, size}];
 i = j = 1;
 For[elem = 0, elem < size^2, elem++,
  data[[i, j]] = elem;
  If[Mod[i + j, 2] == 0,
   If[j < size, j++, i += 2];
   If[i > 1, i--]
   ,
   If[i < size, i++, j += 2];
   If[j > 1, j--];
   ];
  ];
 data
 ]