107. Binary Tree Level Order Traversal II #

Problem #

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]
]

Main Idea of the Problem #

Traverse a tree level by level from bottom to top.

Solution Approach #

This can be implemented using a queue.

Code #

package leetcode

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func levelOrderBottom(root *TreeNode) [][]int {
	tmp := levelOrder(root)
	res := [][]int{}
	for i := len(tmp) - 1; i >= 0; i-- {
		res = append(res, tmp[i])
	}
	return res
}