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.

题解：

class Solution {
public:
  int getDepth(TreeNode *root) {
    if (root == NULL) {
      return 0;
    }
    return 1 + max(getDepth(root->left), getDepth(root->right));
  }
  bool isBalanced(TreeNode* root) {
    if (root == NULL) {
      return true;
    }
    if (abs(getDepth(root->left) - getDepth(root->right)) > 1) {
      return false;
    }
    return isBalanced(root->left) && isBalanced(root->right);
  }
};