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

Author: kamyu104

Python/substrings-that-begin-and-end-with-the-same-letter.py

@@ -0,0 +1,32 @@
+# Time:  O(n)
+# Space: O(1)
+
+import collections
+
+
+class Solution(object):
+    def numberOfSubstrings(self, s):
+        """
+        :type s: str
+        :rtype: int
+        """
+        result = 0
+        cnt = collections.Counter()
+        for c in s:
+            cnt[c] += 1
+            result += cnt[c]
+        return result
+
+
+# Time:  O(n)
+# Space: O(1)
+import collections
+
+
+class Solution(object):
+    def numberOfSubstrings(self, s):
+        """
+        :type s: str
+        :rtype: int
+        """
+        return sum(v*(v+1)//2 for v in collections.Counter(s).itervalues())