// Add Two Numbers

// 跟Add Binary 很类似

// 时间复杂度O(m+n)，空间复杂度O(1)

public class Solution {

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

ListNode dummy = new ListNode(-1); // 头节点

int carry = 0;

ListNode prev = dummy;

for (ListNode pa = l1, pb = l2;

pa != null || pb != null;

pa = pa == null ? null : pa.next,

pb = pb == null ? null : pb.next,

prev = prev.next) {

final int ai = pa == null ? 0 : pa.val;

final int bi = pb == null ? 0 : pb.val;

final int value = (ai + bi + carry) % 10;

carry = (ai + bi + carry) / 10;

prev.next = new ListNode(value); // 尾插法

}

if (carry > 0)

prev.next = new ListNode(carry);

return dummy.next;

}

};