public IList <int> NumIslands2(int rows, int cols, int[,] positions)
{
var dsu = new DSU(rows * cols);
var result = new List <int>();
int GetIndx(int r, int c) => r * cols + c;
for (var i = 0; i < positions.GetLength(0); i++)
{
var r = positions[i, 0];
var c = positions[i, 1];
var indx = GetIndx(r, c);
dsu.Set(indx);
var up = GetIndx(r - 1, c);
if (r - 1 >= 0 && dsu.IsSet(up))
{
dsu.Union(indx, up);
}
var down = GetIndx(r + 1, c);
if (r + 1 < rows && dsu.IsSet(down))
{
dsu.Union(indx, down);
}
var left = GetIndx(r, c - 1);
if (c - 1 >= 0 && dsu.IsSet(left))
{
dsu.Union(indx, left);
}
var right = GetIndx(r, c + 1);
if (c + 1 < cols && dsu.IsSet(right))
{
dsu.Union(indx, right);
}
result.Add(dsu.GetCount());
}
return(result);
}