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// @Title: 二叉树的层序遍历 (Binary Tree Level Order Traversal)
// @Author: 15816537946@163.com
// @Date: 2020-08-30 11:13:24
// @Runtime: 0 ms
// @Memory: 2.9 MB
/**
 * 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(*TreeNode, 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

}