学习笔记

递归不存在明显的性能问题，要拥抱递归

#242 Valid Anagram https://leetcode.com/problems/valid-anagram/

#49 Group Anagram https://leetcode.com/problems/group-anagrams/

#94 Binary Tree Inorder Traversal https://leetcode.com/problems/binary-tree-inorder-traversal/

#144 Binary Tree Preorder Traversal https://leetcode.com/problems/binary-tree-preorder-traversal/

#590 N-ary Tree Postorder Traversal https://leetcode.com/problems/n-ary-tree-postorder-traversal/

#589 N-ary Tree Preorder Traversal https://leetcode.com/problems/n-ary-tree-preorder-traversal/

#347 Top K Frequent Elements https://leetcode.com/problems/top-k-frequent-elements/