Create count-good-meals.cpp

Author: kamyu104

C++/count-good-meals.cpp

@@ -0,0 +1,27 @@
+// Time:  O(n)
+// Space: O(1)
+
+class Solution {
+public:
+    int countPairs(vector<int>& deliciousness) {
+        static const int MOD = 1e9 + 7;
+
+        static const auto& floor_log2_x = [](int x) {
+            return 8 * sizeof(int) - __builtin_clz(x) - 1;
+        };
+        
+        int max_pow = floor_log2_x(*max_element(cbegin(deliciousness),
+                                                cend(deliciousness))) + 1;
+        unordered_map<int, int> cnt;
+        int result = 0;
+        for (const auto& d : deliciousness) {
+            int p = 1;
+            for (int i = 0; i <= max_pow; ++i) {
+                result = (result + cnt[p - d]) % MOD;
+                p <<= 1;
+            }
+            ++cnt[d];
+        }
+        return result;
+    }
+};