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.
解决方案（完美版）：
class Solution {
public boolean isBalanced(TreeNode root) {
return helper(root) != -1;
}
public int helper(TreeNode root){
if(root == null) return 0;
int left = helper(root.left);
int right = helper(root.right);
if(left == -1 || right == -1 || Math.abs(left - right) > 1) return -1;
return Math.max(left, right) + 1;
}
}
自创版本：
class Solution {
public boolean isBalanced(TreeNode root) {
if(root == null){
return true;
}
return
(Math.abs(maxNodeLength(root.left) - maxNodeLength(root.right)) <= 1)
&& isBalanced(root.left)
&& isBalanced(root.right);
}
public int maxNodeLength(TreeNode root){
if(root == null){
return 0;
}
return 1 + Math.max(maxNodeLength(root.left),maxNodeLength(root.right));
}
}