LeetCode 145 Binary Tree Postorder Traversal

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

For example:

Given binary tree 

{1,#,2,3}

,

1
\
2
/
3

return 

[3,2,1]

.

Note: Recursive solution is trivial, could you do it iteratively?

# Definition for a binary tree node
# class TreeNode
# attr_accessor :val, :left, :right
# def initialize(val)
# @val = val
# @left, @right = nil, nil
# end
# end

# @param {TreeNode} root
# @return {Integer[]}
def postorder_traversal(root)
stack = [[false, root]]
ans = []
while not stack.empty?
f, n = stack.pop()
if n
if not f
stack << [true, n] << [false, n.right] << [false, n.left]
else
ans << n.val
end
end
end
ans
end