Tree - 110. Balanced Binary Tree

110. Balanced Binary Tree

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as:

a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4

Return false.

思路：

题目意思是判断一棵二叉树是否是平衡二叉树。而平衡二叉树的定义是一个二叉树_每个节点 _的左右两个子树的高度差的绝对值不超过1，所以，递归的思想，判断一个树是否为平衡二叉树，就看左右子树的高度差和左右子树是否为平衡二叉树。

代码：

go：

/**

 * Definition for a binary tree node.

 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func isBalanced(root *TreeNode) bool {
    if root == nil {
        return true
    }
    
    left := depth(root.Left)
    right := depth(root.Right)
    
    return abs(left, right) <= 1 && isBalanced(root.Left) && isBalanced(root.Right)
}

func depth(node *TreeNode) int {
    if node == nil {
        return 0
    }
    return max(depth(node.Left), depth(node.Right)) + 1
} 

func abs(i, j int) int {
    if i > j {
        return i - j
    }
    return j - i
}

func max(i, j int) int {
    if i > j {
        return i
    } 
    return j 
}