Update 316._Remove_Duplicate_Letters.md

316._Remove_Duplicate_Letters.md

@@ -34,13 +34,13 @@ class Solution(object):
         remaining = collections.defaultdict(int)
         for c in s:
             remaining[c] += 1
-        stack, res = set(), []
+        res, stack = [], set()
         for c in s:
             if c not in stack:
                 while res and res[-1] > c and remaining[res[-1]] > 0:
                     stack.remove(res.pop())
+                res.append(c)
                 stack.add(c)
-                res += c
             remaining[c] -= 1
         return ''.join(res)
 ```