【Lintcode】167. Add Two Numbers

题目地址：

https://www.lintcode.com/problem/add-two-numbers/description

给定两个链表，表示从个位到高位的一个数的十进制表示，求两个数的和并返回同样格式的链表。直接模拟两个数相加即可。代码如下：

public class Solution {
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2
     */
    public ListNode addLists(ListNode l1, ListNode l2) {
        // write your code here
        int cur = 0;
        ListNode dummy = new ListNode(0), prev = dummy;
        while (l1 != null || l2 != null) {
            if (l1 != null) {
                cur += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                cur += l2.val;
                l2 = l2.next;
            }
            prev.next = new ListNode(cur % 10);
            prev = prev.next;
            cur /= 10;
        }
        // 最后的进位如果是1，还需要补一下最高位
        if (cur == 1) {
            prev.next = new ListNode(1);
        }
        
        return dummy.next;
    }
}

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}

时间复杂度 $O(n)$，空间 $O(1)$。