class Solution {
public:
    int ans=INT_MAX;
    int last;
    bool is_first=true;
    int getMinimumDifference(TreeNode* root) {
        dfs(root);
        return ans;
    }
    void dfs(TreeNode* root)
    {
        // 中序遍历
        if(!root)return;
        dfs(root->left);
        if(is_first)is_first=false;
        else ans=min(ans, root->val-last);
        last=root->val;
        dfs(root->right);
    }
};