剑指 Offer 55 - II：平衡二叉树

3
   / \
  9  20
    /  \
   15   7

1
      / \
     2   2
    / \
   3   3
  / \
 4   4

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