// Union Find solution
// great solution
public int NumIslands2(char[][] grid)
{
var m = grid.Length;
var n = grid[0].Length;
var totalLength = m * n;
var uf = new UnionFind(totalLength + 1);
for (int i = 0; i < m; i++)
{
for (int j = 0; j < n; j++)
{
if (grid[i][j] == '1')
{
if (j + 1 < n && grid[i][j + 1] == '1')
{
uf.Union(i * n + j, i * n + j + 1);
}
if (i + 1 < m && grid[i + 1][j] == '1')
{
uf.Union(i * n + j, (i + 1) * n + j);
}
}
else
{
uf.Union(i * n + j, totalLength);
}
}
}
return(uf.Count() - 1);
}
// 261. Graph Valid Tree
// Ridiculously easy with UnionFind
public bool ValidTree(int n, int[][] edges)
{
var uf = new UnionFind(n);
foreach (var e in edges)
{
if (uf.Connected(e[0], e[1]))
{
return(false);
}
uf.Union(e[0], e[1]);
}
return(uf.Count() == 1);
}
