Create unique-length-3-palindromic-subsequences.py

Author: kamyu104

Python/unique-length-3-palindromic-subsequences.py

@@ -0,0 +1,14 @@
+# Time:  O(n)
+# Space: O(1)
+
+class Solution(object):
+    def countPalindromicSubsequence(self, s):
+        """
+        :type s: str
+        :rtype: int
+        """
+        first, last = [len(s)]*26, [-1]*26
+        for i, c in enumerate(s):
+            first[ord(c)-ord('a')] = min(first[ord(c)-ord('a')], i)
+            last[ord(c)-ord('a')] = max(last[ord(c)-ord('a')], i)
+        return sum(len(set(s[i] for i in xrange(first[c]+1, last[c]))) for c in xrange(26))