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: 空叶子高度 = 0 非空叶子高度 = 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 //如果当前节点下左右子树 不平衡 或者 高度 差值 》 1 则 当前节点返回 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 }
