/**

 * Definition for a binary tree node.

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

//BST树中序遍历就是递增的序列，相邻两个数的差值，找最小

class Solution {

public:

    int min_res = INT_MAX;

    TreeNode* pre;

    int getMinimumDifference(TreeNode* root) {

        helper(root);

        return min_res;

    }

    void helper(TreeNode*root){

        if (!root)

            return;

        helper(root->left);  //1

        if (pre)

            min_res = min(min_res, abs(root->val - pre->val));

        pre = root;

        helper(root->right);  //2

    }

};