Create substrings-that-begin-and-end-with-the-same-letter.cpp

Author: kamyu104

C++/substrings-that-begin-and-end-with-the-same-letter.cpp

@@ -0,0 +1,32 @@
+// Time:  O(n)
+// Space: O(1)
+
+class Solution {
+public:
+    long long numberOfSubstrings(string s) {
+        unordered_map<char, int> cnt;
+        int64_t result = 0;
+        for (const auto& c : s) {
+            result += ++cnt[c];
+        }
+        return result;
+    }
+};
+
+// Time:  O(n)
+// Space: O(1)
+class Solution2 {
+public:
+    long long numberOfSubstrings(string s) {
+        unordered_map<char, int64_t> cnt;
+        
+        for (const auto& c : s) {
+            ++cnt[c];
+        }
+        int64_t result = 0;
+        for (const auto& [_, v] : cnt) {
+            result += v * (v + 1) / 2;
+        }
+        return result;
+    }
+};