递归 - Leetcode 110 判断二叉树是否为平衡二叉树

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));

    }

}