404. Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

Example:

    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

# Definition for a binary tree node.
# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val = 0, left = nil, right = nil)
#         @val = val
#         @left = left
#         @right = right
#     end
# end
# @param {TreeNode} root
# @return {Integer}
def sum_of_left_leaves(root)
  @sum = 0
  cal(root, false)
  p @sum
end

def cal(node, is_left_node)
  return if node.nil?
  @sum += node.val if is_left_node && node.left.nil? && node.right.nil?
  cal(node.left, true)
  cal(node.right, false)
end