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You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)Output: 7 -> 0 -> 8Explanation: 342 + 465 = 807.

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        ListNode list = null, s = null;        int sum = 0;        while (l1 != null || l2 != null) {            sum /= 10;            if (l1 != null) {                sum += l1.val;                l1 = l1.next;            }            if (l2 != null) {                sum += l2.val;                l2 = l2.next;            }            if (list == null) {                list = new ListNode(sum % 10);                s = list;            } else {                ListNode node = new ListNode(sum % 10);                s.next = node;                s = s.next;            }        }        if (sum / 10 == 1) {            s.next = new ListNode(1);        }        return list;    }}

Debug code in playground:

/* ----------------------------------- *  WARNING: * ----------------------------------- *  Your code may fail to compile *  because it contains public class *  declarations. *  To fix this, please remove the *  "public" keyword from your class *  declarations. *//** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        ListNode list = null, s = null;        int sum = 0;        while (l1 != null || l2 != null) {            sum /= 10;            if (l1 != null) {                sum += l1.val;                l1 = l1.next;            }            if (l2 != null) {                sum += l2.val;                l2 = l2.next;            }            if (list == null) {                list = new ListNode(sum % 10);                s = list;            } else {                ListNode node = new ListNode(sum % 10);                s.next = node;                s = s.next;            }        }        if (sum / 10 == 1) {            s.next = new ListNode(1);        }        return list;    }}public class MainClass {    public static int[] stringToIntegerArray(String input) {        input = input.trim();        input = input.substring(1, input.length() - 1);        if (input.length() == 0) {          return new int[0];        }            String[] parts = input.split(",");        int[] output = new int[parts.length];        for(int index = 0; index < parts.length; index++) {            String part = parts[index].trim();            output[index] = Integer.parseInt(part);        }        return output;    }        public static ListNode stringToListNode(String input) {        // Generate array from the input        int[] nodeValues = stringToIntegerArray(input);            // Now convert that list into linked list        ListNode dummyRoot = new ListNode(0);        ListNode ptr = dummyRoot;        for(int item : nodeValues) {            ptr.next = new ListNode(item);            ptr = ptr.next;        }        return dummyRoot.next;    }        public static String listNodeToString(ListNode node) {        if (node == null) {            return "[]";        }            String result = "";        while (node != null) {            result += Integer.toString(node.val) + ", ";            node = node.next;        }        return "[" + result.substring(0, result.length() - 2) + "]";    }        public static void main(String[] args) throws IOException {        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));        String line;        while ((line = in.readLine()) != null) {            ListNode l1 = stringToListNode(line);            line = in.readLine();            ListNode l2 = stringToListNode(line);                        ListNode ret = new Solution().addTwoNumbers(l1, l2);                        String out = listNodeToString(ret);                        System.out.print(out);        }    }}

这个不要理所当然想成了头插法，看到测试代码才知道是尾插法，返回的ListNode也是需要尾插法的。

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