[369. Plus One Linked List] #leetcode

369_1005.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode plusOne(ListNode head) {
        if (head == null) return null;
        
        ListNode pre = null;
        ListNode cur = head;
        // reverse linkedlist to the end node
        while (cur != null && cur.next != null) {
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        
        // add one
        int carry = 1;
            
        // reverse back
        while (pre != null) {
            cur.val += carry;
            carry = cur.val / 10;
            cur.val = cur.val % 10;
            
            ListNode beforePre = pre.next;
            pre.next = cur;
            cur = pre;
            pre = beforePre;
            
        }
        
        if (carry > 0) {
            cur.val += carry;
            carry = cur.val / 10;
            cur.val = cur.val % 10;
            
            if (carry > 0) {
                ListNode newHead = new ListNode(carry);
                newHead.next = head;
                head = newHead;
            }
        }
        
        return head;
    }
}


class Solution1 {
    public ListNode plusOne(ListNode head) {
        if( DFS(head) == 0){
            return head;
        }else{
            ListNode newHead = new ListNode(1);
            newHead.next = head;
            return newHead;
        }
    }

    public int DFS(ListNode head){
        if(head == null) return 1;

        int carry = DFS(head.next);

        int val = head.val + carry;
        head.val = val%10;
        return val/10;
    }
}