530 783 买一送一 在中序遍历上简单修改 C++

class Solution {
public:
    int minDiffInBST(TreeNode* root) {
        stack<TreeNode*> stk;
        int pre = -1, ans = INT_MAX;

        while (!stk.empty() || root != nullptr) {

            while (root != nullptr) {
                stk.push(root);
                root = root->left;
            }

            root = stk.top();
            stk.pop();
            if (pre != -1)
                ans = min(root->val - pre, ans);
                
            pre = root->val;
            root = root->right;
        }

        return ans;

    }
};