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NumberOfIslands.java
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NumberOfIslands.java
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class NumberOfIslands {
public List<Integer> numIslands2(int m, int n, int[][] positions) {
List<Integer> res = new ArrayList<>();
UnionFind uf = new UnionFind(m * n);
boolean[][] island = new boolean[m][n];
for (int[] pos : positions) {
int i = p[0];
int j = p[1];
if (island[i][j]) {
res.add(uf.count());
continue;
}
uf.count++;
island[i][j] = true;
int id = i * n + j;
if (i - 1 >= 0 && island[i - 1][j]) {
uf.union(id, id - n);
}
if (j - 1 >= 0 && island[i][j - 1]) {
uf.union(id, id - 1);
}
if (i + 1 < m && island[i + 1][j]) {
uf.union(id, id + n);
}
if (j + 1 < n && island[i][j + 1]) {
uf.union(id, id + 1);
}
res.add(uf.count);
}
return res;
}
private class UnionFind {
private int[] ids;
private int[] sizes;
public int count;
UnionFind(int n) {
this.ids = new int[n];
this.sizes = new int[n];
for (int i = 0; i < n; i++) {
ids[i] = i;
sizes = 1;
}
}
private int getRoot(int a) {
int root = a;
while (root != ids[root]) {
root = ids[root];
}
while (a != root) {
int next = ids[a];
ids[a] = root;
a = next;
}
return root;
}
public void union(int a, int b) {
int roota = getRoot(a);
int rootb = getRoot(b);
if (roota == rootb) {
return;
}
count--;
if (sizes[roota] >= sizes[rootb]) {
ids[rootb] = roota;
sizes[roota] += sizes[rootb];
} else {
ids[roota] = rootb;
sizes[rootb] += sizes[roota];
}
}
}
}