[Cracking the Coding Interview] 4.4 Check Balanced

Implement a function to check if a binary tree is balanced. For the purpose of this question, a balanced tree is defined to be a tree such that the heights of the two subtrees of any node never differ by more than one.

这道题让我们检查一棵树是不是平衡的，根据题目平衡二叉树的定义我们可以对于每个node递归的检测一下它的substrees是否符合这个条件。见如下的代码：

height函数递归的计算出一棵树的高度，is_balanced?函数递归的判断每个node的子树是不是平衡。

class TreeNode
  attr_accessor :val, :left, :right

  def initialize(val)
    @val = val
    @left = nil
    @right = nil
  end
end

def is_balanced?(root)
  return true if root.nil?

  if (height(root.left) - height(root.right)).abs <= 1
    return is_balanced?(root.left) && is_balanced?(root.right)
  else
    return false
  end
end

def height(root)
  return 0 if root.nil?

  left_height = height(root.left)
  right_height = height(root.right)

  return [left_height, right_height].max + 1
end