diff --git a/261-Graph-Valid-Tree.py b/261-Graph-Valid-Tree.py
index 0f5e5f36b..8b48a71b4 100644
--- a/261-Graph-Valid-Tree.py
+++ b/261-Graph-Valid-Tree.py
@@ -1,4 +1,4 @@
-# Problem is free on Lintcode
+# Problem is free on Lintcode https://www.lintcode.com/problem/178/
class Solution:
"""
@param n: An integer
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