将二叉搜索树变平衡

class Solution {
public:
    //对二叉树进行递归
    void helper(TreeNode* root, vector<int>& nums){
        if(root==nullptr) return;
        helper(root->left, nums);
        nums.push_back(root->val);
        helper(root->right, nums);
    }
    //对二叉树进行中序遍历
    vector<int> midSort(TreeNode* root){
        vector<int> nums;
        helper(root, nums);
        return nums;
    }
    //构建平衡二叉树的递归函数
    TreeNode* helper1(vector<int>& nums, int left, int right){
        if(left>right) return nullptr;
        int mid = (left+right)/2;
        TreeNode* root = new TreeNode(nums[mid]);
        root->left = helper1(nums, left, mid-1);
        root->right = helper1(nums, mid+1, right);
        return root;
    }
    //构建平衡二叉树
    TreeNode* balanceBST(TreeNode* root) {
        vector<int> nums = midSort(root);
        return helper1(nums, 0, nums.size()-1);
    }
};