add folders for week 7

Author: DaleSeo

binary-tree-level-order-traversal/README.md

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+- 문제: https://leetcode.com/problems/binary-tree-level-order-traversal/
+- 풀이: https://www.algodale.com/problems/binary-tree-level-order-traversal/

lowest-common-ancestor-of-a-binary-search-tree/README.md

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+- 문제: https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/
+- 풀이: https://www.algodale.com/problems/lowest-common-ancestor-of-a-binary-search-tree/

remove-nth-node-from-end-of-list/README.md

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+- 문제: https://leetcode.com/problems/remove-nth-node-from-end-of-list/
+- 풀이: https://www.algodale.com/problems/remove-nth-node-from-end-of-list/

reorder-list/README.md

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+- 문제: https://leetcode.com/problems/reorder-list/
+- 풀이: https://www.algodale.com/problems/reorder-list/

validate-binary-search-tree/README.md

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+- 문제: https://leetcode.com/problems/validate-binary-search-tree/
+- 풀이: https://www.algodale.com/problems/validate-binary-search-tree/