add folders for week 8

Author: DaleSeo

combination-sum/README.md

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+- 문제: https://www.algodale.com/problems/combination-sum/
+- 풀이: https://www.algodale.com/problems/combination-sum/

construct-binary-tree-from-preorder-and-inorder-traversal/README.md

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+- 문제: https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
+- 풀이: https://www.algodale.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/

implement-trie-prefix-tree/README.md

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+- 문제: https://leetcode.com/problems/implement-trie-prefix-tree/
+- 풀이: https://www.algodale.com/problems/implement-trie-prefix-tree/

kth-smallest-element-in-a-bst/README.md

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+- 문제: https://leetcode.com/problems/kth-smallest-element-in-a-bst/
+- 풀이: https://www.algodale.com/problems/kth-smallest-element-in-a-bst/

word-search/README.md

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+- 문제: https://leetcode.com/problems/word-search/
+- 풀이: https://www.algodale.com/problems/word-search/