369. Plus One Linked List

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode plusOne(ListNode head) {
      // put two numbers into stack separtely
      // when pop them out, we add two numbers and keep a carry for the next
      // digit, and push every sum into the stack 
      
      // after we done pushing the sum into the stack, 
      // we can pop them out and link them into a linked list 
      ListNode l2 = new ListNode(1);
      
      Deque<ListNode> stack1 = new ArrayDeque<>();
      Deque<ListNode> stack2 = new ArrayDeque<>();
      
      while(head != null) {
        stack1.push(head);
        head = head.next;
      }
      while( l2 != null) {
        stack2.push(l2);
        l2 = l2.next;
      }
      
      int carry = 0;
      Deque<Integer> stack = new ArrayDeque<>();
      while( !stack1.isEmpty() || !stack2.isEmpty()){
        int num1 = stack1.size() == 0 ? 0 : stack1.pop().val;
        int num2 = stack2.size() == 0 ? 0 : stack2.pop().val;
        
        int sum = carry + num1 + num2;
        carry = sum / 10;
        sum = sum % 10;
        stack.push(sum);
      }
      if( carry != 0){
        stack.push(carry);
      }
      
      
      ListNode dummy = new ListNode(0);
      ListNode cur = dummy;
      while(!stack.isEmpty()){
        int number = stack.pop();
        ListNode newListNode = new ListNode(number);
        cur.next = newListNode;
        cur = cur.next;
      }
      return dummy.next;
    }
}

 

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