【Leetcode】101. Symmetric Tree

Description

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3
But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3
Note:
Bonus points if you could solve it both recursively and iteratively.

Recursion

# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val)
#         @val = val
#         @left, @right = nil, nil
#     end
# end

# @param {TreeNode} root
# @return {Boolean}
def is_symmetric(root)
  return true if root.nil?
  mirror = lambda do |a, b|
    return true if a.nil? && b.nil?
    return false if a.nil? ^ b.nil? || a.val != b.val
    mirror.call(a.left, b.right) && mirror.call(a.right, b.left)
  end

  mirror.call(root.left, root.right)
end

Iterative

# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val)
#         @val = val
#         @left, @right = nil, nil
#     end
# end

# @param {TreeNode} root
# @return {Boolean}
def is_symmetric(root)
    return true if root.nil?

    tree = []
    tree << root.left
    tree << root.right

    while !tree.empty?
        left  = tree.shift
        right = tree.pop

        next if left.nil? && right.nil?
        return false if left.nil? || right.nil?
        return false if left.val != right.val

        tree.unshift(left.left, left.right)
        tree.push(right.left, right.right)
    end

    true
end