Create friends-of-appropriate-ages.cpp

Author: kamyu104

C++/friends-of-appropriate-ages.cpp

@@ -0,0 +1,27 @@
+// Time:  O(a^2 + n), a is the number of ages,
+//                    n is the number of people
+// Space: O(a)
+
+class Solution {
+public:
+    int numFriendRequests(vector<int>& ages) {
+        unordered_map<int, int> count;
+        for (const auto &age : ages) {
+            ++count[age];
+        }
+        int result = 0;
+        for (const auto &a: count) {
+            for (const auto &b: count) {
+                if (request(a.first, b.first)) {
+                    result += a.second * (b.second - (a.first == b.first ? 1 : 0));
+                }
+            }
+        }
+        return result;
+    }
+
+private:
+    bool request(int a, int b) {
+        return 0.5 * a + 7 < b && b <= a;
+    }
+};