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
平衡二叉搜索树（Self-balancing binary search tree）又被称为AVL树（有别于AVL算法），且具有以下性质：
它是一 棵空树或它的左右两个子树的高度差的绝对值不超过1，并且左右两个子树都是一棵平衡二叉树。
public bool IsBalanced(TreeNode root) { if (root == null) { return true; } int maxLeftDepth = MaxDepth(root.left); int maxRightDepth = MaxDepth(root.right); bool flag1 = Math.Abs(maxLeftDepth - maxRightDepth) <= 1; bool flag2 = IsBalanced(root.left); bool flag3 = IsBalanced(root.right); return flag1 && flag2 && flag3; } public int MaxDepth(TreeNode root) { int depth; if (root == null) { depth = 0; } else { depth = 1; TreeNode left = root.left; TreeNode right = root.right; if (left != null || right != null) { int leftDepth = MaxDepth(left); int rightDepth = MaxDepth(right); depth = depth + Math.Max(leftDepth, rightDepth); } } return depth; }
原文：https://www.cnblogs.com/chucklu/p/10689027.html