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// @Title: 二叉树的层序遍历 (Binary Tree Level Order Traversal)
// @Author: 15816537946@163.com
// @Date: 2020-06-25 07:15:36
// @Runtime: 4 ms
// @Memory: 6.1 MB
/*
 * @lc app=leetcode.cn id=102 lang=golang
 *
 * [102] 二叉树的层次遍历
 *
 * https://leetcode-cn.com/problems/binary-tree-level-order-traversal/description/
 *
 * algorithms
 * Medium (58.64%)
 * Likes:    275
 * Dislikes: 0
 * Total Accepted:    44.7K
 * Total Submissions: 76K
 * Testcase Example:  '[3,9,20,null,null,15,7]'
 *
 * 给定一个二叉树,返回其按层次遍历的节点值。 (即逐层地,从左到右访问所有节点)。
 *
 * 例如:
 * 给定二叉树: [3,9,20,null,null,15,7],
 *
 * ⁠   3
 * ⁠  / \
 * ⁠ 9  20
 * ⁠   /  \
 * ⁠  15   7
 *
 *
 * 返回其层次遍历结果:
 *
 * [
 * ⁠ [3],
 * ⁠ [9,20],
 * ⁠ [15,7]
 * ]
 *
 *
 */
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func levelOrder(root *TreeNode) [][]int {
	var ret [][]int
	var travel func(node *TreeNode, flag int)
	travel = func(node *TreeNode, flag int) {
		if node == nil {
			return
		}
		if flag >=len(ret) {
			ret = append(ret,[]int{})
		}
		ret[flag] = append(ret[flag], node.Val)
		travel(node.Left, flag+1)
		travel(node.Right, flag+1)
	}
	travel(root, 0)
	return ret
}