102. Binary Tree Level Order Traversal

Medium
​
Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).
Example 1:
​
​
Input: root = [3,9,20,null,null,15,7]
Output:
[[3],[9,20],[15,7]]
Example 2:
Input: root = [1]
Output:
[[1]]
Example 3:
Input: root = []
Output:
[]
Constraints:
  • The number of nodes in the tree is in the range [0, 2000].
  • -1000 <= Node.val <= 1000

解題

/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
func levelOrder(root *TreeNode) [][]int {
res := make([][]int, 0)
​
var helper func(*TreeNode, int)
helper = func(root *TreeNode, level int) {
if root == nil {
return
}
​
if level > len(res) {
res = append(res, []int{root.Val})
} else {
res[level - 1] = append(res[level - 1], root.Val)
}
​
helper(root.Left, level + 1)
helper(root.Right, level + 1)
}
​
helper(root, 1)
​
return res
}