Two Lists Sum Advanced

Source

Given two numbers represented by two linked lists, write a function that returns sum list.
The sum list is linked list representation of addition of two input numbers.

Example

Input:
  First  List: 5->6->3  // represents number 563
  Second List: 8->4->2  // represents number 842
Output
  Resultant list: 1->4->0->5  // represents number 1405

Challenge

Not allowed to modify the lists.
Not allowed to use explicit extra space.

题解1 - 反转链表

在题 Two Lists Sum 的基础上改了下数位的表示方式，前者低位在前，高位在后，这个题的高位在前，低位在后。很自然地可以联想到先将链表反转，而后再使用 Two Lists Sum 的解法。

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverse(ListNode head) {
        ListNode prev = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }

    public ListNode addLists(ListNode l1, ListNode l2) {
        l1 = reverse(l1);
        l2 = reverse(l2);
        ListNode dummy = new ListNode(0);
        ListNode curr = dummy;
        int carry = 0;

        while ((l1 != null) || (l2 != null) || (carry != 0)) {
            int l1_val = (l1 != null) ? l1.val : 0;
            int l2_val = (l2 != null) ? l2.val : 0;
            int sum = carry + l1_val + l2_val;
        // update carry
            carry = sum / 10;
            curr.next = new ListNode(sum % 10);

            curr = curr.next;
            if (l1 != null) l1 = l1.next;
            if (l2 != null) l2 = l2.next;
        }

        return reverse(dummy.next);
    }
}

源码分析

参考 Two Lists Sum

复杂度分析

参考 Two Lists Sum