0107-binary-tree-level-order-traversal-ii.adoc

107. Binary Tree Level Order Traversal II

{leetcode}/problems/binary-tree-level-order-traversal-ii/[LeetCode - Binary Tree Level Order Traversal II^]

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

   3
  / \
 9  20
   /  \
  15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

解题分析

分层访问，然后再把顺序反转过来。

思考题

尝试一下递归的解法！

参考资料

1. 107二叉树的层次遍历II（队列法和递归法） - 二叉树的层次遍历 II - 力扣（LeetCode）

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

link:{sourcedir}/_0107_BinaryTreeLevelOrderTraversalII.java[role=include]

link:{sourcedir}/_0107_BinaryTreeLevelOrderTraversalII_2.java[role=include]

相比第一次解，第二次解直接在链表前面添加元素即可，这样就无需反转了。