diff --git a/LeetCode/305.cpp b/LeetCode/305.cpp
new file mode 100644
index 0000000..9e4c46d
--- /dev/null
+++ b/LeetCode/305.cpp
@@ -0,0 +1,83 @@
+
+class DisjointSet
+{
+public:
+    int* parent;
+    int countRoot;
+
+    DisjointSet(int size) {
+        parent = new int[size + 1];
+        memset(parent, 0, sizeof(int) * (size + 1));
+        countRoot = 0;
+    }
+
+    int find(int id) {
+        if (parent[id] && parent[id] != id) {
+            parent[id] = find(parent[id]);
+        }
+
+        // printf("parent of %d is %d\n", id, parent[id]);
+        return parent[id];
+    }
+
+    void setParent(int child, int upper) {
+        if (upper == child) {
+            parent[child] = child;
+            countRoot++;
+        }
+        else {
+            int childRoot = find(child);
+            int upperRoot = find(upper);
+            if (childRoot != upperRoot) countRoot--;
+
+            // printf("set parent of %d to %d\n", childRoot, upperRoot);
+            parent[childRoot] = upperRoot;
+        }
+    }
+};
+
+class Solution {
+public:
+    vector<int> numIslands2(int m, int n, vector<vector<int>>& positions) {
+        int sz = m * n;
+        DisjointSet dset(sz);
+        vector<int> results;
+
+        for (auto& pr : positions) {
+            int line = pr[0];
+            int col = pr[1];
+            int id = line * n + col + 1;
+
+            if (dset.parent[id]) {
+                results.push_back(dset.countRoot);
+                continue;
+            }
+
+            dset.setParent(id, id);
+
+            if (line > 0) {
+                int upid = (line - 1) * n + col + 1;
+                if (dset.parent[upid]) dset.setParent(upid, id);
+            }
+
+            if (line < m - 1) {
+                int downid = (line + 1) * n + col + 1;
+                if(dset.parent[downid]) dset.setParent(downid, id);
+            }
+
+            if (col > 0) {
+                int leftid = line * n + col;
+                if (dset.parent[leftid]) dset.setParent(leftid, id);
+            }
+
+            if (col < n - 1) {
+                int rightid = line * n + col + 2;
+                if (dset.parent[rightid]) dset.setParent(rightid, id);
+            }
+
+            results.push_back(dset.countRoot);
+        }
+
+        return results;
+    }
+};