Added solution

Author: avidLearnerInProgress

sort_by_bits.cpp

@@ -0,0 +1,14 @@
+class Solution {
+public:
+    vector<int> sortByBits(vector<int>& arr) {
+        
+        
+        sort(arr.begin(), arr.end(), [](const int& A, const int &B){
+            
+            int countA = __builtin_popcount(A), countB = __builtin_popcount(B);
+            return countA == countB ? A < B : countA < countB;
+        });
+        
+        return arr;
+    }
+};