import java.util.ArrayDeque; /** * Use Elementary Math to solve the problem. attention: * *

1. Use dummy head when using LinkNode. It's convenient because you don't need to judge whether it's head. *

2. Simplify your code. (when l1 is null or l2 is null, use 0 to substitute!) *

3. judge whether carry is 1 when it ends. * * @author LBW */ public class AddTwoNumbers { /** * this is my initial version */ @SuppressWarnings("Duplicates") @Deprecated public ListNode addTwoNumbers(ListNode l1, ListNode l2) { ListNode head = null, current = null; int carry = 0; while (l1 != null && l2 != null) { int val1 = l1.val; int val2 = l2.val; int sum = val1 + val2 + carry; if (sum >= 10) carry = 1; else carry = 0; if (head == null) { head = new ListNode(sum % 10); current = head; } else { current.next = new ListNode(sum % 10); current = current.next; } l1 = l1.next; l2 = l2.next; } while (l1 != null) { int sum = l1.val + carry; if (sum >= 10) carry = 1; else carry = 0; current.next = new ListNode(sum % 10); current = current.next; l1 = l1.next; } while (l2 != null) { int sum = l2.val + carry; if (sum >= 10) carry = 1; else carry = 0; current.next = new ListNode(sum % 10); current = current.next; l2 = l2.next; } // if still has carry. like 5+5=10 if (carry == 1) current.next = new ListNode(1); return head; } /** * this is referenced by standard solution. */ public ListNode standardAddTwoNumbers(ListNode l1, ListNode l2) { ListNode dummyHead = new ListNode(0); ListNode current = dummyHead; int carry = 0; while (l1 != null || l2 != null) { int p = l1 != null ? l1.val : 0; int q = l2 != null ? l2.val : 0; int sum = p + q + carry; carry = sum / 10; current.next = new ListNode(sum % 10); current = current.next; if (l1 != null) l1 = l1.next; if (l2 != null) l2 = l2.next; } if (carry == 1) current.next = new ListNode(1); return dummyHead.next; } public static void main(String[] args) { AddTwoNumbers addTwoNumbers = new AddTwoNumbers(); addTwoNumbers.addTwoNumbers(null, null); addTwoNumbers.standardAddTwoNumbers(null, null); } }