230. Kth Smallest Element in a BST
230. Kth Smallest Element in a BST
題目求，如何在一個平衡二元樹中找出第 k
Tree
430. Flatten a Multilevel Doubly Linked List
430. Flatten a Multilevel Doubly Linked List
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547. Number of Provinces
547. Number of Provinces
這個題目是有 n 個島嶼，其中島嶼可能會相連，我們要知道是不是相連的方式是題目有給定一個 n
250. Count Univalue Subtrees
250. Count Univalue Subtrees
# Definition for a binary tree node.
99. Recover Binary Search Tree
99. Recover Binary Search Tree
# Definition for a binary tree
1135. Connecting Cities With Minimum Cost
1135. Connecting Cities With Minimum Cost
這一題題目給的標籤是中等難度，不過應該算是偏向困難的中等題目，算是進階版的廣度優先搜索題目。
這一個題目我會建議先去想題目要問的是什麼，
701. Insert into a Binary Search Tree
701. Insert into a Binary Search Tree
如果是要在一個 BST 中插入一個點，
112. Path Sum
112. Path Sum
這個題目很像是 N Sum 的題目，不過是從根節點開始，找到是不是可以從根節點到葉節點之間，所有的值加起來剛好和目標相同。
426. Convert Binary Search Tree to Sorted Doubly Linked List
426. Convert Binary Search Tree to Sorted Doubly Linked List
1379. Find a Corresponding Node of a Binary Tree in a Clone of That Tree
1379. Find a Corresponding Node of a Binary Tree in