按层来遍历二叉树。思路非常简单，用 FIFO 队列来解决就好。

下面的代码以这颗树为测试用例：

       1
      / \
     2   3
    / \   \
   4   5   6
  /       / \
 7       8   9
              \
              10

package main

import "fmt"

type TreeNode struct {
	Val   int
	Left  *TreeNode
	Right *TreeNode
}

func levelOrderTraversal(root *TreeNode) []int {
	queue := []*TreeNode{root}
	answer := []int{}

	for len(queue) != 0 {
		curr := queue[0]
		answer = append(answer, curr.Val)
		queue = queue[1:]

		if curr.Left != nil {
			queue = append(queue, curr.Left)
		}
		if curr.Right != nil {
			queue = append(queue, curr.Right)
		}
	}
	return answer
}

func main() {
	root := &TreeNode{1,
		&TreeNode{2,
			&TreeNode{4, &TreeNode{7, nil, nil}, nil},
			&TreeNode{5, nil, nil}},
		&TreeNode{3, nil,
			&TreeNode{6,
				&TreeNode{8, nil, nil},
				&TreeNode{9,
					nil,
					&TreeNode{10, nil, nil},
				},
			}}}
	fmt.Println(levelOrderTraversal(root))
}