Time: 9 ms (66.90%), Space: 45.8 MB (62.15%) - LeetHub

Author: Akcps

0305-number-of-islands-ii/0305-number-of-islands-ii.java

@@ -0,0 +1,113 @@
+class Solution {
+    public List<Integer> numIslands2(int m, int n, int[][] positions) {
+        int x[] = { -1, 1, 0, 0 };
+        int y[] = { 0, 0, -1, 1 };
+        
+        UnionFind dsu = new UnionFind(m * n);
+        
+        List<Integer> answer = new ArrayList<>();
+
+        for (int[] position: positions) {
+            
+            int landPosition = position[0] * n + position[1];
+            
+            dsu.addLand(landPosition);
+
+            for (int i = 0; i < 4; i++) {
+                
+                int neighborX = position[0] + x[i];
+                int neighborY = position[1] + y[i];
+                
+                int neighborPosition = neighborX * n + neighborY;
+                
+                if (neighborX >= 0 && neighborX < m && neighborY >= 0 && neighborY < n && dsu.isLand(neighborPosition)) {
+                    
+                    dsu.union(landPosition, neighborPosition);
+    
+                }
+            }
+            
+            answer.add(dsu.numberOfIslands());
+            
+        }
+        
+        return answer;
+    }
+}
+
+class UnionFind {
+    
+    int[] parent;
+    int[] rank;
+    
+    int count;
+
+    public UnionFind(int size) {
+        
+        parent = new int[size];
+        rank = new int[size];
+        
+        for (int i = 0; i < size; i++) {
+            
+            parent[i] = -1;
+            
+        }
+        
+        count = 0;
+    }
+
+    public void addLand(int x) {
+        
+        if (parent[x] >= 0) {
+            return;
+        }   
+        
+        parent[x] = x;
+        count++;
+        
+    }
+
+    public boolean isLand(int x) {
+        
+        if (parent[x] >= 0) {
+            return true;
+        } else {
+            return false;
+        }
+        
+    }
+
+    int numberOfIslands() {
+        return count;
+    }
+
+    public int find(int x) {
+        
+        if (parent[x] != x) {
+           parent[x] = find(parent[x]); 
+        }
+        
+        return parent[x];
+        
+    }
+
+    public void union(int x, int y) {
+        
+        int xset = find(x);
+        int yset = find(y);
+        
+        if (xset == yset) {
+            return;
+        } else if (rank[xset] < rank[yset]) {
+            parent[xset] = yset;
+        } else if (rank[xset] > rank[yset]) {
+            parent[yset] = xset;
+        } else {
+            parent[yset] = xset;
+            rank[xset]++;
+        }
+        
+        count--;
+        
+    }
+}