https://leetcode.com/problems/balanced-binary-tree/description/
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.


bal. binary tree:

    3      3: max(1,2) + 1  = 3
 

   / \
  9  20    9: 1 20:2 (max 1, 1) + 1 = 2
    /  \
   15   7  both 1

not bal. binary tree:
    3          3-1 > 1 not bal. return -1
   / \
  9  20        9: 1 ;  20: max(1,2) + 1 = 3
    /  \
   15   7      15: 1  7: 2
         \
          9     9: 1

 1 public boolean isBalanced(TreeNode root) {
 2         if
 
 (root == null) return true;
 3         return getHeight(root) != -1 ;
 4     }
 5     //get current node height: -1 代表不平衡
 6     private int getHeight(TreeNode root){
 7         //base case: 空葉子高度 = 0 非空葉子高度 = 1
 8         if (root == null) return 0 ;
 9         // post order
10         int l = getHeight(root.left) ;
 
11         int r = getHeight(root.right) ;
12         int diff = Math.abs(l-r) ;
13         //如果當前節點下左右子樹 不平衡 或者 高度 差值 》 1 則 當前節點返回 不平衡 -1
14         if (l == -1 || r == -1 || diff > 1) {
15             return - 1 ;
16         }
17         //如果平衡，返回 正常高度
18         return Math.max(l,r) + 1 ;
19     }

LC.110. Balanced Binary Tree