Create friend-circles.cpp

Author: kamyu104

C++/friend-circles.cpp

@@ -0,0 +1,48 @@
+// Time:  O(n^2)
+// Space: O(n)
+
+class Solution {
+public:
+    int findCircleNum(vector<vector<int>>& M) {
+        UnionFind circles(M.size());
+        for (int i = 0; i < M.size(); ++i) {
+            for (int j = 0; j < M[i].size(); ++j) {
+                if (M[i][j] && i != j) {
+                    circles.union_set(i, j);
+                }
+            }
+        }
+        return circles.size();
+    }
+
+private:
+    class UnionFind {
+        public:
+            UnionFind(const int n) : set_(n), count_(n) {
+                iota(set_.begin(), set_.end(), 0);
+            }
+
+            int find_set(const int x) {
+               if (set_[x] != x) {
+                   set_[x] = find_set(set_[x]);  // Path compression.
+               }
+               return set_[x];
+            }
+
+            void union_set(const int x, const int y) {
+                int x_root = find_set(x), y_root = find_set(y);
+                if (x_root != y_root) {
+                    set_[min(x_root, y_root)] = max(x_root, y_root);
+                    --count_;
+                }
+            }
+
+            int size() const {
+                return count_;
+            }
+
+        private:
+            vector<int> set_;
+            int count_;
+    };
+};