diff --git a/LeetCode/827.cpp b/LeetCode/827.cpp
new file mode 100644
index 0000000..87f5cd4
--- /dev/null
+++ b/LeetCode/827.cpp
@@ -0,0 +1,153 @@
+
+class DisjointSet
+{
+public:
+    int* parent;
+    int countRoot;
+
+    DisjointSet(int size) {
+        parent = new int[size];
+        memset(parent, 0xFF, sizeof(int) * size);
+        countRoot = 0;
+    }
+
+    ~DisjointSet() {
+        delete[] parent;
+    }
+
+    int find(int id) {
+        if (parent[id] != -1 && 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 {
+            if (parent[child] == -1) {
+                parent[child] = child;
+                countRoot++;
+            }
+            if (parent[upper] == -1) {
+                parent[upper] = upper;
+                countRoot++;
+            }
+
+            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:
+    int largestIsland(vector<vector<int>>& grid) {
+        int n = grid.size();
+        DisjointSet dset = DisjointSet(n * n);
+
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                if (!grid[i][j]) continue;
+
+                if (dset.find(i * n + j) == -1) {
+                    dset.setParent(i * n + j, i * n + j);
+                }
+
+                if (i > 0 && grid[i - 1][j]) {
+                    dset.setParent((i - 1) * n + j, i * n + j);
+                }
+
+                if (i < n - 1 && grid[i + 1][j]) {
+                    dset.setParent((i + 1) * n + j, i * n + j);
+                }
+
+                if (j > 0 && grid[i][j - 1]) {
+                    dset.setParent(i * n + j - 1, i * n + j);
+                }
+
+                if (j < n - 1 && grid[i][j + 1]) {
+                    dset.setParent(i * n + j + 1, i * n + j);
+                }
+            }
+        }
+
+        unordered_map<int, int> areaMap;
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                if (!grid[i][j]) continue;
+
+                int root = dset.find(i * n + j);
+                auto iter = areaMap.find(root);
+                if (iter != areaMap.end()) {
+                    iter->second++;
+                }
+                else {
+                    areaMap.insert_or_assign(root, 1);
+                }
+            }
+        }
+
+        int maxArea = 0;
+        for (auto iter = areaMap.begin(); iter != areaMap.end(); iter++) {
+            maxArea = max(maxArea, iter->second);
+        }
+
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                if (grid[i][j]) continue;
+                unordered_set<int> usedRoot;
+
+                int totalArea = 1;
+                if (i > 0 && grid[i-1][j]) {
+                    int root = dset.find((i - 1) * n + j);
+                    if (usedRoot.find(root) == usedRoot.end()) {
+                        usedRoot.insert(root);
+                        int area = areaMap.find(root)->second;
+                        totalArea += area;
+                    }
+                }
+
+                if (i < n - 1 && grid[i + 1][j]) {
+                    int root = dset.find((i + 1) * n + j);
+                    if (usedRoot.find(root) == usedRoot.end()) {
+                        usedRoot.insert(root);
+                        int area = areaMap.find(root)->second;
+                        totalArea += area;
+                    }
+                }
+
+                if (j > 0 && grid[i][j - 1]) {
+                    int root = dset.find(i * n + j - 1);
+                    if (usedRoot.find(root) == usedRoot.end()) {
+                        usedRoot.insert(root);
+                        int area = areaMap.find(root)->second;
+                        totalArea += area;
+                    }
+                }
+
+                if (j < n - 1 && grid[i][j + 1]) {
+                    int root = dset.find(i * n + j + 1);
+                    if (usedRoot.find(root) == usedRoot.end()) {
+                        usedRoot.insert(root);
+                        int area = areaMap.find(root)->second;
+                        totalArea += area;
+                    }
+                }
+
+                maxArea = max(maxArea, totalArea);
+            }
+        }
+
+        return maxArea;
+    }
+};