Create smallest-subsequence-of-distinct-characters.py

Author: kamyu104

Python/smallest-subsequence-of-distinct-characters.py

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+# Time:  O(n)
+# Space: O(1)
+
+import collections
+
+
+class Solution(object):
+    def smallestSubsequence(self, text):
+        """
+        :type text: str
+        :rtype: str
+        """
+        count = collections.Counter(text)
+
+        lookup, stk = set(), []
+        for c in text:
+            if c not in lookup:
+                while stk and stk[-1] > c and count[stk[-1]]:
+                    lookup.remove(stk.pop())
+                stk += c
+                lookup.add(c)
+            count[c] -= 1
+        return "".join(stk)