链表数加上1

https://leetcode.com/problems/plus-one-linked-list/

Given a non-negative number represented as a singly linked list of digits, plus one to the number.

The digits are stored such that the most significant digit is at the head of the list.

Example:
Input:
1->2->3

Output:
1->2->4

ListNode* plusOne(ListNode* head) {
    auto dummy = new ListNode(0);
    dummy->next = head;
    // 找最后一个!=9（不进位）的位置
    ListNode *found;
    for (auto p = dummy; p; p = p->next) {
        if (p->val != 9) found = p;
    }
    // found位置加1，后面位置全变0
    found->val += 1;
    for (auto p = found->next; p; p = p->next) {
        p->val = 0;
    }
    // 如果dummy后全是9，found=dummy，dummy->val==1
    if (dummy->val != 0) return dummy;
    auto ans = dummy->next;
    delete dummy;
    return ans;
}