/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> nums;
    void track_back(TreeNode* cur)
    {
        if(cur == nullptr) return;
        track_back(cur->left);
        nums.push_back(cur->val);
        track_back(cur->right);
        return;
    }
    TreeNode* make_tree(vector<int> &nums , int left ,int right)
    {
        if(left > right) return nullptr;
        int mid = left + (right - left)/2;
        TreeNode* new_root = new TreeNode(nums[mid]);
        new_root->left = make_tree(nums , left ,mid-1);
        new_root->right = make_tree(nums , mid+1 ,right);
        return new_root;
    }
    TreeNode* balanceBST(TreeNode* root) {
        track_back(root);
        return make_tree(nums,0,nums.size()-1);
    }
};