Solution.c

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */

struct TreeNode* bstToGst(struct TreeNode* root) {
    struct TreeNode* cur = root;
    int sum = 0;
    while (cur) {
        if (!cur->right) {
            sum += cur->val;
            cur->val = sum;
            cur = cur->left;
        } else {
            struct TreeNode* next = cur->right;
            while (next->left && next->left != cur) {
                next = next->left;
            }
            if (!next->left) {
                next->left = cur;
                cur = cur->right;
            } else {
                next->left = NULL;
                sum += cur->val;
                cur->val = sum;
                cur = cur->left;
            }
        }
    }
    return root;
}