# Number of Islands II

**Hard**

A 2d grid map of`m`rows and`n`columns is initially filled with water. We may perform anaddLandoperation which turns the water at position (row, col) into a land. Given a list of positions to operate,**count the number of islands after each addLand operation**. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

**Example:**

```
Input:
 m = 3, n = 3, positions = [[0,0], [0,1], [1,2], [2,1]]

Output:
 [1,1,2,3]
```

**Explanation:**

Initially, the 2d grid`grid`is filled with water. (Assume 0 represents water and 1 represents land).

```
0 0 0
0 0 0
0 0 0
```

Operation #1: addLand(0, 0) turns the water at grid\[0]\[0] into a land.

```
1 0 0
0 0 0   Number of islands = 1
0 0 0
```

Operation #2: addLand(0, 1) turns the water at grid\[0]\[1] into a land.

```
1 1 0
0 0 0   Number of islands = 1
0 0 0
```

Operation #3: addLand(1, 2) turns the water at grid\[1]\[2] into a land.

```
1 1 0
0 0 1   Number of islands = 2
0 0 0
```

Operation #4: addLand(2, 1) turns the water at grid\[2]\[1] into a land.

```
1 1 0
0 0 1   Number of islands = 3
0 1 0
```

**Follow up:**

Can you do it in time complexity O(k log mn), where k is the length of the`positions`?

## Analysis

思路：

使用**并查集**，每个格子作为一个节点。当一个格子变成岛屿，和它的四个邻居依次连接，相当于在图中加四条边

其他方法似乎都会TLE，并查集似乎是唯一能过OJ的方法。

## Solution

Union Find

Use rank to do path compression

```java
class Solution {
    class UnionFind {
        int count; // # of connected components
        int[] parent;
        int[] rank;

        public UnionFind(char[][] grid) { // for problem 200
            count = 0;
            int m = grid.length;
            int n = grid[0].length;
            parent = new int[m * n];
            rank = new int[m * n];
            for (int i = 0; i < m; ++i) {
                for (int j = 0; j < n; ++j) {
                    if (grid[i][j] == '1') {
                        parent[i * n + j] = i * n + j;
                        ++count;
                    }
                    rank[i * n + j] = 0;
                }
            }
        }

        public UnionFind(int N) { // for problem 305 and others
            count = 0;
            parent = new int[N];
            rank = new int[N];
            for (int i = 0; i < N; ++i) {
                parent[i] = -1;
                rank[i] = 0;
            }
        }

        public boolean isValid(int i) { // for problem 305
            return parent[i] >= 0;
        }

        public void setParent(int i) {
            parent[i] = i;
            ++count;
        }

        public int find(int i) { // path compression
            if (parent[i] != i) parent[i] = find(parent[i]);
            return parent[i];
        }

        public void union(int x, int y) { // union with rank
            int rootx = find(x);
            int rooty = find(y);
            if (rootx != rooty) {
                if (rank[rootx] > rank[rooty]) {
                    parent[rooty] = rootx;
                } else if (rank[rootx] < rank[rooty]) {
                    parent[rootx] = rooty;
                } else {
                    parent[rooty] = rootx; rank[rootx] += 1;
                }
                --count;
            }
        }

        public int getCount() {
            return count;
        }
    }

    public List<Integer> numIslands2(int m, int n, int[][] positions) {
        List<Integer> ans = new ArrayList<>();
        UnionFind uf = new UnionFind(m * n);

        int[] dr = {0, 1, 0, -1};
        int[] dc = {1, 0, -1, 0};

        for (int[] pos : positions) {
            int r = pos[0], c = pos[1];
            int id = r * n + c;
            uf.setParent(id);

            for (int k = 0; k < 4; k++) {
                int nr =  r + dr[k];
                int nc =  c + dc[k];
                int nid = nr * n + nc;

                if (nr >= 0 && nr < m && nc >= 0 && nc < n) {
                    if (uf.isValid(nid)) {
                        uf.union(nid, id);
                    }
                }
            }
            ans.add(uf.getCount());
        }
        return ans;
    }
}
```


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