vector<int> numIslands2(int m, int n, vector<pair<int, int>>& positions) {
if (positions.size() == 0) {
return vector<int>();
}
vector<int> res;
vector<int> nodes(m * n, -1);
vector<pair<int, int>> dir = { { -1, 0 }, { 1, 0 }, { 0, -1 }, { 0, 1 } };
int count = 0;
for (int i = 0; i < positions.size(); ++i) {
pair<int, int> pos = positions[i];
int rootIndex = pos.first * n + pos.second;
nodes[rootIndex] = rootIndex;
count++;
for (pair<int, int> d : dir) {
int x = pos.first + d.first;
int y = pos.second + d.second;
int nextIndex = x * n + y;
if (x < 0 || x >= m || y < 0 || y >= n || nodes[nextIndex] == -1) {
continue;
}
int nextRootIndex = findIsland(nodes, nextIndex);
if (nextRootIndex != rootIndex) {
nodes[rootIndex] = nextRootIndex;
rootIndex = nextRootIndex;
count--;
}
}
res.push_back(count);
}
return res;
}
vector<int> numIslands2(int m, int n, vector<pair<int, int>>& positions) {
vector<int> ret;
if(m<=0 || n <=0) return ret;
int numIsland = 0; //count the number of islands
vector<int> roots(m*n, -1); //one island = one tree
vector<vector<int>> dirs = {{-1,0},{1,0},{0,1},{0,-1}}; //four directions
for(pair<int,int> p:positions){
int root = n * p.first + p.second;
roots[root] = root; //add a new island
numIsland++;
for(vector<int> d:dirs){
int x = p.first + d[0];
int y = p.second + d[1];
int newId = n * x + y;
if(x<0 || x>=m || y<0 || y>=n || roots[newId]==-1){
continue;
}
//not -1, there is already an island
int rootId = findIsland(roots, newId);
if(root != rootId){
roots[root] = rootId;
root = rootId; //union this to neighbor island
numIsland--;
}
}
ret.push_back(numIsland);
}
return ret;
}
