diff --git a/graph-valid-tree/README.md b/graph-valid-tree/README.md
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+- 문제
+	- 유료: https://leetcode.com/problems/graph-valid-tree/
+	- 무료: https://www.lintcode.com/problem/178/
+- 풀이: https://www.algodale.com/problems/graph-valid-tree/
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diff --git a/house-robber-ii/README.md b/house-robber-ii/README.md
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+- 문제: https://leetcode.com/problems/house-robber-ii/
+- 풀이: https://www.algodale.com/problems/house-robber-ii/
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diff --git a/house-robber/README.md b/house-robber/README.md
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+- 문제: https://leetcode.com/problems/house-robber/
+- 풀이: https://www.algodale.com/problems/house-robber/
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diff --git a/longest-palindromic-substring/README.md b/longest-palindromic-substring/README.md
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+- 문제: https://leetcode.com/problems/longest-palindromic-substring/
+- 풀이: https://www.algodale.com/problems/longest-palindromic-substring/
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diff --git a/number-of-connected-components-in-an-undirected-graph/README.md b/number-of-connected-components-in-an-undirected-graph/README.md
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+- 문제
+	- 유료: https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/
+	- 무료: https://www.lintcode.com/problem/3651/
+- 풀이: https://www.algodale.com/problems/number-of-connected-components-in-an-undirected-graph/
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