783. 二叉搜索树节点最小距离

C代码：二叉树

int min;
int pre;

int dfs(struct TreeNode* root) {
    if (root == NULL) {
        return;
    }
    dfs(root->left);
    if (pre != -1) {
        min = fmin(min, root->val - pre);
    }
    pre = root->val;  // 中序遍历
    dfs(root->right);
}

int minDiffInBST(struct TreeNode* root){
    min = INT_MAX;
    pre = -1;
    dfs(root);
    return min;
}