L 369. Plus One Linked List ----M

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

You may assume the integer do not contain any leading zero, except the number 0 itself.

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]

A:

核心技巧就是加了一个 dummy header, 然后递归调用

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* plusOne(ListNode* head) {
        ListNode * dummy = new ListNode(0);
        dummy->next = head;
        
        helper(dummy);
        if(dummy->val == 1){            
            return dummy;
        }else{
            delete dummy;
            return head;
        }
    }
private:
    bool helper(ListNode* head){
        if(not head->next){
            int carry = (head->val + 1) /10;
            head->val = (head->val + 1) % 10;
            return carry ==1;
        }else{
            bool hasCarry = helper(head->next);
            if(hasCarry){                
                int carry = (head->val + 1) /10;
                head->val = (head->val + 1) % 10;
                return carry ==1;                
            }else{
                return false;
            }
        }        
    }
};