public List<Integer> numIslands2(int m, int n, int[][] positions) {
if (m < 0 || n < 0 || positions == null) {
throw new IllegalArgumentException();
}
List<Integer> result = new ArrayList<Integer>();
UnionFind uf = new UnionFind();
int[] x = new int[] {-1, 1, 0, 0};
int[] y = new int[] {0, 0, -1, 1};
for (int[] p : positions) {
int i = p[0];
int j = p[1];
int cur = id(i, j, n);
uf.add(cur);
for (int k = 0; k < x.length; k++) {
if (valid(i + x[k], j + y[k], m, n)) {
int neighbor = id(i + x[k], j + y[k], n);
uf.union(cur, neighbor);
}
}
result.add(uf.count);
}
return result;
}
