102. Binary Tree Level Order Traversal

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]
]

# @lc code=start
using LeetCode

function level_order(root::Union{Nothing, TreeNode{Int}})
    isnothing(root) && return Int[]
    q = TreeNode{Int}[]
    q2 = TreeNode{Int}[]
    res = Vector{Int}[]
    push!(q, root)
    while !isempty(q)
        push!(res, Int[])
        while !isempty(q)
            fst = popfirst!(q)
            push!(res[end], fst.val)
            !isnothing(fst.left) && push!(q2, fst.left)
            !isnothing(fst.right) && push!(q2, fst.right)
        end
        q, q2 = q2, q
    end
    return res
end
# @lc code=end

level_order (generic function with 1 method)

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