617. Merge Two Binary Trees

先序遍历，递归 先序遍历，非递归

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669. Trim a Binary Search Tree

先序遍历，递归

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637. Average of Levels in Binary Tree

BFS算法 BFS DFS算法 DFS

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104. Maximum Depth of Binary Tree

DFS BFS

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226. Invert Binary Tree

BFS非递归 递归

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606. Construct String from Binary Tree

先序遍历，递归 先序遍历，递归 先序遍历，递归 先序遍历，递归

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538. Convert BST to Greater Tree

按右中左的顺序遍历

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404. Sum of Left Leaves

递归 非递归

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563. Binary Tree Tilt

递归

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100. Same Tree

非递归 递归

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543. Diameter of Binary Tree

递归算法 递归实现

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671. Second Minimum Node In a Binary Tree

递归

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108. Convert Sorted Array to Binary Search Tree

递归方法 递归