add code of 200

Author: Li Li

top-interview-questions/200. Number of Islands.cs

@@ -0,0 +1,79 @@
+// recursion with dfs, easy solution but not typical for these bfs problem
+public class Solution {
+    private int m, n;
+    public int NumIslands(char[,] grid) {
+        m = grid.GetLength(0);
+        n = grid.GetLength(1);
+        if (m == 0 || n == 0) return 0;
+        int count = 0;
+        for (int i = 0; i < m; i++) {
+            for (int j = 0; j < n; j++) {
+                if (grid[i,j] == '1') {
+                    count++;
+                    DfsFill(grid, i, j);
+                }
+            }
+        }
+        return count;
+    }
+    private void DfsFill(char[,] grid, int i, int j) {
+        // fill the point with label '1' and inside range
+        if (i >=0 && i < m && j >=0 && j < n && grid[i,j] == '1') {
+            grid[i, j] = '0';
+            DfsFill(grid, i + 1, j);
+            DfsFill(grid, i - 1, j);
+            DfsFill(grid, i, j + 1);
+            DfsFill(grid, i, j - 1);
+        }
+    }
+}
+
+// BFS, recursion
+public class Solution {
+    private int m, n;
+    public int NumIslands(char[,] grid) {
+        m = grid.GetLength(0);
+        n = grid.GetLength(1);
+        if (m == 0 || n == 0) return 0;
+        int count = 0;
+        for (int i = 0; i < m; i++) {
+            for (int j = 0; j < n; j++) {
+                if (grid[i,j] == '1') {
+                    count++;
+                    BfsFill(grid, i, j);
+                }
+            }
+        }
+        return count;
+    }
+    class Point {
+        public int x, y;
+        public Point(int x, int y) {
+            this.x = x;
+            this.y = y;
+        }
+    }
+    private void BfsFill(char[,] grid, int x, int y) {
+        int[] deltaX = { 1, -1, 0, 0 };
+        int[] deltaY = { 0, 0, 1, -1 };
+
+        Queue<Point> q = new Queue<Point>();
+        q.Enqueue(new Point(x, y));
+        grid[x, y] = '0';
+        while (q.Count > 0) {
+            Point point1 = q.Dequeue();
+            for (int i = 0; i < 4; i++) {
+                Point adj = new Point(
+                    point1.x + deltaX[i],
+                    point1.y + deltaY[i]
+                );
+                if (adj.x >= 0 && adj.x < m
+                    && adj.y >= 0 && adj.y < n 
+                    && grid[adj.x, adj.y] == '1') {
+                    grid[adj.x, adj.y] = '0';
+                    q.Enqueue(adj);
+                }
+            }
+        }
+    }
+}