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Plus One Linked List

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Description

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

Solution

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* plusOne(ListNode* head) {
        if (head == NULL) return head;
        
        ListNode* cur = new ListNode(0);
        ListNode* ret = cur;
        cur->next = head;
        
        while(cur->next) {
            stack<ListNode*> st;
            st.push(cur); //The digit before the leading 9
            while(cur->next && cur->next->val == 9) {
                cur = cur->next;
                st.push(cur);
            }
            if(cur->next == NULL) {
                //The last digit is 9
                while(st.size() > 1) {
                    ListNode* item = st.top();
                    st.pop();
                    item->val = 0;
                }
                st.top()->val += 1;
                return ret->val ? ret : ret->next;
            }
            //Find the next 9
            while(cur->next && cur->next->val != 9) cur = cur->next;
        }
        //The last digit is not 9
        cur->val += 1;
        return ret->next;
    }
};