783. 二叉搜索树节点最小距离 二叉树中序遍历

给你一个二叉搜索树的根节点 root ，返回 树中任意两不同节点值之间的最小差值 。
https://leetcode-cn.com/problems/minimum-distance-between-bst-nodes/

/**
 * 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:
    int ans = INT_MAX;
    int pre = -1;

    void dfs(TreeNode* root) {
        if (!root) {
            return;
        }

        dfs(root->left);

        if (pre == -1){
            pre = root->val;
        }
        else {
            ans = min(ans, root->val - pre);
            pre = root->val;
        }

        dfs(root->right);
    }
    int minDiffInBST(TreeNode* root) {
        dfs(root);
        return ans;
    }
};