144. Binary Tree Preorder Traversal

Given the root of a binary tree, return the preorder traversal of its nodes ' values.

Example 1:

Input: root = [1,null,2,3]
Output: [1,2,3]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Example 4:

Input: root = [1,2]
Output: [1,2]

Example 5:

Input: root = [1,null,2]
Output: [1,2]

Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

Follow up:

Recursive solution is trivial, could you do it iteratively?

# @lc code=start
using LeetCode
preorder_traversal(::Nothing) = Int[]
function preorder_traversal(root::TreeNode)::Vector{Int}
    res, stack = Int[], []
    while !isempty(stack) || !isnothing(root)
        while !isnothing(root)
            push!(stack, root)
            push!(res, root.val) ## preorder search
            root = root.left
        end
        root = pop!(stack)
        root = root.right
    end
    return res
end

##### code template for inorder traversal and preorder traversal #####
# traversal(::Nothing) = Int[]
# function traversal(root::TreeNode)::Vector{Int}
#     res, stack = Int[], []
#     while !isempty(stack) || !isnothing(root)
#         while !isnothing(root)
#             push!(stack, root)
#             ## preorder traversal: put codes here
#             root = root.left
#         end
#         root = pop!(stack)
#         ## inorder traversal: put codes here
#         root = root.right
#     end
#     res
# end

# @lc code=end

preorder_traversal (generic function with 2 methods)

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