Number of Islands II

Posted 2021-03-21 flagyuri

Given a n,m which means the row and column of the 2D matrix and an array of pair A( size k). Originally, the 2D matrix is all 0 which means there is only sea in the matrix. The list pair has k operator and each operator has two integer A[i].x, A[i].y means that you can change the grid matrix[A[i].x][A[i].y] from sea to island. Return how many island are there in the matrix after each operator.

0 is represented as the sea, 1 is represented as the island. If two 1 is adjacent, we consider them in the same island. We only consider up/down/left/right adjacent.

Example

Example 1:

Input: n = 4, m = 5, A = [[1,1],[0,1],[3,3],[3,4]]
Output: [1,1,2,2]
Explanation:
0.  00000
    00000
    00000
    00000
1.  00000
    01000
    00000
    00000
2.  01000
    01000
    00000
    00000
3.  01000
    01000
    00000
    00010
4.  01000
    01000
    00000
    00011

Example 2:

Input: n = 3, m = 3, A = [[0,0],[0,1],[2,2],[2,1]]
Output: [1,1,2,2]

思路使用并查集，并查集的实现可以利用Point结构体, 也可以将二维坐标转换成一维。注意使用类的时候需重写equals方法和hashCode方法。

　　对于每一次操作(x, y), 如果(x, y)的上下左右都是0, 那么计数器加一; 如果不全为0, 则:

1. 并查集查询其四周的1所属的集合, 假设它们属于 k 个不同的集合
2. 计数器减去 k-1
3. 将这 k 个集合, 连同 (x, y), 合并 

/**
 * Definition for a point.
 * class Point {
 *     int x;
 *     int y;
 *     Point() { x = 0; y = 0; }
 *     Point(int a, int b) { x = a; y = b; }
 * }
 */



public class Solution {
    /**
     * @param n: An integer
     * @param m: An integer
     * @param operators: an array of point
     * @return: an integer array
     */
     
class MyPoint{
       int x;
       int y;
       MyPoint() { x = 0; y = 0; }
       MyPoint(int a, int b) { x = a; y = b; }

    @Override
    public boolean equals(Object obj) {
        if (obj == this)
            return true;
        return this.x == ((MyPoint) obj).x && this.y == ((MyPoint) obj).y;
    }
    
    @Override
    public int hashCode() {
            return x * 10 + y;
        }
}
class UnionFind{
    HashMap<MyPoint, MyPoint> father = new HashMap<>();
    UnionFind(int n, int m) {
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                MyPoint p = new MyPoint(i, j);
                father.put(p, p);
            }
        }
    }
    
    MyPoint find(MyPoint p) {
        MyPoint point = p;
        while (point != father.get(point)) {
            point = father.get(point);
        }
        MyPoint root = point;
        while (p != father.get(p)) {
            MyPoint tmp = father.get(p);
            father.put(tmp, root);
            p = tmp;
        }
        return root;
    }
    
    void union(MyPoint p1, MyPoint p2) {
        MyPoint root1 = find(p1);
        MyPoint root2 = find(p2);
        if (root1 != root2) {
            father.put(root1, root2);
        }
    }
}
    public List<Integer> numIslands2(int n, int m, Point[] operators) {
        List<Integer> ans = new ArrayList<>();
        if (operators == null || operators.length == 0) {
            return ans;
        }
        int[] dx = {0, -1, 0, 1};
        int[] dy = {1, 0, -1, 0};
        int[][] island = new int[n][m];
        UnionFind uf = new UnionFind(n, m);
        int count = 0;
        for (int i = 0; i < operators.length; i++) {
            int x = operators[i].x;
            int y = operators[i].y;
            if (island[x][y] != 1) {
                island[x][y] = 1;
                count++;
                MyPoint p = new MyPoint(x, y);
                for (int j = 0; j < 4; j++) {
                    int nx = x + dx[j];
                    int ny = y + dy[j];
                    if (0 <= nx && nx < n && 0 <= ny && ny < m && island[nx][ny] == 1) {
                        MyPoint np = new MyPoint(nx, ny);
                        MyPoint root1 = uf.find(p);
                        MyPoint root2 = uf.find(np);
                        if (root1 != root2) {
                            count--;
                            uf.union(p, np);
                        }
                    }
                }
            }
            ans.add(count);
        }
          return ans;
    }
}