/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
const int64_t INF = numeric_limits::max();

class Solution {
public:
    int minDiffInBST(TreeNode* root) {
        int64_t min_diff = INF;
        int64_t pred_val = INF;
        minDiff(root, pred_val, min_diff);


        //min_diff = min( min_diff, abs(prev_val -

        return min_diff;
    }

private:
    void minDiff(TreeNode* root, int64_t& pred_val, int64_t& min_diff) {
        if (!root) return ;

        minDiff(root->left, pred_val, min_diff);
        min_diff = min(min_diff, abs(pred_val - root->val));
        pred_val = root->val;
        minDiff(root->right, pred_val, min_diff);
    }
};