Add Two Numbers

题目描述：

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 -> 8
Explanation: 342 + 465 = 807.

输入样例其实为两个字符串，如：[2,4,3] [5,6,4]，输出为：[7,0,8]

解题思路：

1. 将ListNode链先逆序一下再转换成数字类型，根据两个数字相加之后的结果转换成ListNode链即为输出。
   但是输入有可能很长，超出了int的范围，例如：[1] [9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9]。所以这种方法不可取
2. 通过仔细观察输入输出样例，发现它和正常的加法很类似。两个数正常相加，从最低位开始相加，如果低位相加之后的数值大于等于10，则向高位进一位。而题目给出的加法是从最高为开始相加，如果高位相加之后的数值大于等于10，则向低位进一位。
   在计算的过程中，两个ListNode链从最高位（即ListNode链头部）对应相加，直到某一条链到头为止，然后再依次添加上最长链的每一位。
   java代码如下：

/* -----------------------------------
 *  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.
 */


import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

class ListNode {
    int val;
    ListNode next;

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

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode list = new ListNode(0);
        ListNode r = list;//标记指针，指list头部
        list.val = l1.val + l2.val;
        if(list.val >= 10) {
            list.next = new ListNode(1);
            list.val -= 10;
        }
        while(l1.next != null){
            if(l2.next != null){
                l1 = l1.next;
                l2 = l2.next;
                if(list.next != null){
                    list.next.val += l1.val + l2.val;
                }else {
                    list.next = new ListNode(l1.val + l2.val);
                }
            } else {//l1比l2长
                l1 = l1.next;
                if(list.next != null){
                    list.next.val += l1.val;
                } else {
                    list.next = new ListNode(l1.val);
                }
            }
            list = list.next;
            if(list.val >= 10) {
                list.next = new ListNode(1);
                list.val -= 10;
            }
        }
        while(l2.next != null){//l2比l1长
            l2 = l2.next;
            if(list.next != null){
                list.next.val += l2.val;
            } else {
                list.next = new ListNode(l2.val);
            }
            list = list.next;
            if(list.val >= 10) {
                list.next = new ListNode(1);
                list.val -= 10;
            }
        }
        return r;
    }
}

public class AddTwoNumbers {
    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);
        }
    }
}

测试结果

Add Two Numbers题目链接