Binary Tree Level Order Traversal

描述

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


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



return its level order traversal as:
[
  [3],
  [9,20],
  [15,7]
]

题解

package algorithms

import "github.com/ljun20160606/leetcode/algorithms"

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */

type TreeNode = algorithms.TreeNode

func levelOrder(root *TreeNode) [][]int {
	var r [][]int
	helper(&r, root, 0)
	return r
}

func helper(r *[][]int, node *TreeNode, level int) {
	if node == nil {
		return
	}

	if level >= len(*r) {
		*r = append(*r, []int{})
	}

	(*r)[level] = append((*r)[level], node.Val)

	i := level + 1
	helper(r, node.Left, i)
	helper(r, node.Right, i)
}