add folders for week2

Author: DaleSeo

invert-binary-tree/README.md

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

linked-list-cycle/README.md

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

merge-two-sorted-lists/README.md

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+- 문제: https://leetcode.com/problems/merge-two-sorted-lists/
+- 풀이: https://www.algodale.com/problems/merge-two-sorted-lists/

reverse-linked-list/README.md

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

valid-parentheses/REAMEE.md

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+- 문제: https://leetcode.com/problems/valid-parentheses/
+- 풀이: https://www.algodale.com/problems/valid-parentheses/