Tree Levels From the Bottom Up

Medium

You are given the root of a binary tree. Group the node values by level (nodes at the same depth) and return the levels starting from the deepest one and ending with the root's level. Within a level, values go from left to right, the way the nodes appear when the tree is drawn. For an empty tree, return an empty list.

Example 1

root=tree_node([4, 8, -2, None, 5, 1]) → [[5, 1], [8, -2], [4]]

The lowest level holds 5 (right child of 8) and 1 (left child of -2), above it are 8 and -2, and at the top is the root 4.

Example 2

root=tree_node([7, 3]) → [[3], [7]]

First the only leaf 3, then the root 7.

Example 3

root=tree_node([]) → []

An empty tree has no levels.

Constraints

0 ≤ number of nodes ≤ 2000
-1000 ≤ Node.val ≤ 1000

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Topics

Tree Breadth-First Search Binary Tree

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Tree Rows by Depth Medium Snake Through the Tree Levels Medium The Tree Seen From the Right Medium Tree Columns, Top Down Medium First Value on the Lowest Row Medium

Input

root=tree_node([4, 8, -2, None, 5, 1])

Expected

[[5, 1], [8, -2], [4]]

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