cpp: add «1915. Number of Wonderful Substrings»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cpp/number-of-wonderful-substrings.cpp

@@ -0,0 +1,29 @@
+#include <cstdint>
+#include <string>
+#include <unordered_map>
+
+class Solution {
+    static const std::uint32_t BIT = 1;
+
+  public:
+    long long wonderfulSubstrings(const std::string &word) {
+        long long result = 0;
+
+        std::unordered_map<std::uint32_t, long long> freqs;
+        freqs[0] = 1;
+
+        std::uint32_t mask = 0;
+        for (char c : word) {
+            mask ^= BIT << (c - 'a');
+
+            result += freqs[mask];
+            ++freqs[mask];
+
+            for (auto odd_c = 0; odd_c < 10; ++odd_c) {
+                result += freqs[mask ^ (BIT << odd_c)];
+            }
+        }
+
+        return result;
+    }
+};