Leetcode tree题型总结_leetcode tree 总结

对我来说,Tree 是 Leetcode中最简单的题，因为Tree的题型比较单一，方法也比较固定。

98. Validate Binary Search Tree https://leetcode.com/problems/validate-binary-search-tree/我觉得这道题和Symmetric Tree比较像都是考察树的结构，要判断这个需要判断左右节点，采用从底到上，以Root为开始，depth-first-search执行得到对所有的结果的判断。

96. Unique Binary Search Trees

https://leetcode.com/problems/unique-binary-search-trees/ 这道题有公式

95. Unique Binary Search Trees II???

101. Symmetric Tree https://leetcode.com/problems/symmetric-tree/

129. Sum Root to Leaf Numbers https://leetcode.com/problems/sum-root-to-leaf-numbers/先序遍历

100. Same Tree

https://leetcode.com/problems/same-tree/

99. Recover Binary Search Tree https://leetcode.com/problems/recover-binary-search-tree/？？？

116. Populating Next Right Pointers in Each Node

https://leetcode.com/problems/populating-next-right-pointers-in-each-node/？？

117. Populating Next Right Pointers in Each Node II

https://leetcode.com/problems/populating-next-right-pointers-in-each-node-ii/

111. Minimum Depth of Binary Tree

https://leetcode.com/problems/minimum-depth-of-binary-tree/

同path sum, 利用先序遍历记录树的信息。

104. Maximum Depth of Binary Tree

https://leetcode.com/problems/maximum-depth-of-binary-tree/

同样利用先序遍历，记录树的信息。

236. Lowest Common Ancestor of a Binary Tree

https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/

遍历左右子树，如果左右孩子不为空，说明遍历到，则认为找到了最小的孩子。

235. Lowest Common Ancestor of a Binary Search Tree

https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/

如果要找的节点一个是大于当前节点，一个小于当前节点，则现在的节点就是要找的值，否则移向同一侧。

230. Kth Smallest Element in a BST

https://leetcode.com/problems/kth-smallest-element-in-a-bst/

Binary Search Tree的中序遍历的结果是从小到大的顺序，所以记录现在遍历到的节点的值，如果等于Target 就返回该节点。

110. Balanced Binary Tree

https://leetcode.com/problems/balanced-binary-tree/

记录树的高度，并判断左右子树的高度的差的绝对值，如果大于1则不是Balanced Binary tree.从顶往底依次遍历。

144. Binary Tree Preorder Traversal

https://leetcode.com/problems/binary-tree-preorder-traversal/

145. Binary Tree Postorder Traversal

https://leetcode.com/problems/binary-tree-postorder-traversal/

94. Binary Tree Inorder Traversal

https://leetcode.com/problems/binary-tree-inorder-traversal/

三个顺序的Tree的traverse顺序，只是在help函数中访问节点的顺序不同。

102. Binary Tree Level Order Traversal

https://leetcode.com/problems/binary-tree-level-order-traversal/

利用queue进行层序遍历

107. Binary Tree Level Order Traversal II

倒序的层序遍历，每次把结果加到Result的第一个的位置