public class Solution {

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

if (l1 == null) return l2;

if (l2 == null) return l1;

int cb = 0;

ListNode pre = null;

ListNode p = l1;

while (p != null && l2 != null) {

cb = computeSum(p, l2, cb);

pre = p;

p = p.next;

l2 = l2.next;

}

if (l2 != null) {

pre.next = l2;

p = l2;

}

while (p != null) {

cb = computeSum(p, null, cb);

pre = p;

p = p.next;

}

if (cb == 1) {

pre.next = new ListNode(1);

}

return l1;

}

private int computeSum(ListNode x, ListNode y, int cb) {

if (x == null) throw new IllegalArgumentException("xxx");

int t = 0;

if (y != null) t = y.val;

x.val = x.val + t + cb;

cb = 0;

if (x.val >= 10) {

cb = 1;

x.val %= 10;

}

return cb;

}

}