cc150 Chapter 2 | Linked Lists 2.5 add two integer LinkedList, return LinkedList as a sum

2.5 You have two numbers represented by a linked list, where each node contains a single digit. The digits are stored in reverse order, such that the 1’s digit is at the head of the list. Write a function that adds the two numbers and returns the sum as a linked list. EXAMPLE Input: (3 -> 1 -> 5) + (5 -> 9 -> 2) Output: 8 -> 0 -> 8

给两个整型链表，每个节点包含一个位数，这些位数是反向存放的，也就是个位数在链表首部，编写程序，对这两个整数求和，并用链表形式返回结果。

进阶：

假设这些位数是正向存放的，请再做一遍。

1.自己的思路是：取每个list的第一个，相加（随时取余，相除），放入新建一个列表的最后一个。

 public class newAdd5 {
     public static LinkedList<Integer> sum1(LinkedList<Integer> l1,
             LinkedList<Integer> l2) {
         LinkedList<Integer> l3 = new LinkedList<Integer>();
         if (l1.isEmpty())
             return l2;
         if (l2.isEmpty())
             return l1;
         int carry = 0;
         while (!l1.isEmpty() && !l2.isEmpty()) {
             int sum = 0;
             sum = (l1.peek() + l2.peek() + carry) % 10;
             carry = (l1.poll() + l2.poll() + carry) / 10;
             l3.addFirst(sum);
         }
         while (!l1.isEmpty() && l2.isEmpty()) {
             int sum = 0;
             sum = (l1.peek() + carry) % 10;
             carry = (l1.poll() + carry) / 10;
             l3.addFirst(sum);
         }
         while (!l2.isEmpty() && l1.isEmpty()) {
             int sum = 0;
             sum = (l2.peek() + carry) % 10;
             carry = (l2.poll() + carry) / 10;
             l3.addFirst(sum);
         }
         return l3;

     }

     // follow up
     public static LinkedList<Integer> sum2(LinkedList<Integer> l1,
             LinkedList<Integer> l2) {
         java.util.Collections.reverse(l1);
         java.util.Collections.reverse(l2);
         LinkedList<Integer> l3 = new LinkedList<Integer>();
         if (l1.isEmpty())
             return l2;
         if (l2.isEmpty())
             return l1;
         int carry = 0;
         while (!l1.isEmpty() && !l2.isEmpty()) {
             int sum = 0;
             sum = (l1.peek() + l2.peek() + carry) % 10;
             carry = (l1.poll() + l2.poll() + carry) / 10;
             l3.addFirst(sum);
         }
         while (!l1.isEmpty() && l2.isEmpty()) {
             int sum = 0;
             sum = (l1.peek() + carry) % 10;
             carry = (l1.poll() + carry) / 10;
             l3.addFirst(sum);
         }
         while (!l2.isEmpty() && l1.isEmpty()) {
             int sum = 0;
             sum = (l2.peek() + carry) % 10;
             carry = (l2.poll() + carry) / 10;
             l3.addFirst(sum);
         }
         return l3;

     }

     public static void main(String[] args) {
         Random rd = new Random();
         LinkedList<Integer> list1 = new LinkedList<Integer>();
         LinkedList<Integer> list2 = new LinkedList<Integer>();
         int n1 = rd.nextInt(5);
         for (int i = 0; i < n1; i++) {
             list1.add(rd.nextInt(10));
         }
         int n2 = rd.nextInt(6);
         for (int i = 0; i < n2; i++) {
             list2.add(rd.nextInt(10));
         }
         System.out.println(list1.toString());
         System.out.println(list2.toString());
         System.out.println("After Adding: ");
         System.out.println(sum1(list1, list2).toString());
         LinkedList<Integer> list3 = new LinkedList<Integer>();
         LinkedList<Integer> list4 = new LinkedList<Integer>();
         for (int i = 0; i < n1; i++) {
             list3.add(rd.nextInt(10));
         }
         for (int i = 0; i < n2; i++) {
             list4.add(rd.nextInt(10));
         }
         System.out.println("Follow Up: ");
         System.out.println(list3.toString());
         System.out.println(list4.toString());
         System.out.println(sum2(list3, list4).toString());
     }
 }

然后在网上看的方法更简洁：

public class Sum5 {
    public static void main(String[] args) {
        Random rd = new Random();
        LinkedList<Integer> list1 = new LinkedList<Integer>();
        LinkedList<Integer> list2 = new LinkedList<Integer>();
        int n1 = rd.nextInt(5);
        for (int i = 0; i < n1; i++) {
            list1.add(rd.nextInt(10));
        }
        int n2 = rd.nextInt(6);
        for (int i = 0; i < n2; i++) {
            list2.add(rd.nextInt(10));
        }
        System.out.println(list1.toString());
        System.out.println(list2.toString());
        System.out.println("After Adding: ");
        System.out.println(addReverse(list1, list2).toString());
        LinkedList<Integer> list3 = new LinkedList<Integer>();
        LinkedList<Integer> list4 = new LinkedList<Integer>();
        for (int i = 0; i < n1; i++) {
            list3.add(rd.nextInt(10));
        }
        for (int i = 0; i < n2; i++) {
            list4.add(rd.nextInt(10));
        }
        System.out.println("Follow Up: ");
        System.out.println(list3.toString());
        System.out.println(list4.toString());
        System.out.println(addForward(list3, list4).toString());
    }

    private static LinkedList<Integer> addReverse(LinkedList<Integer> list1,
            LinkedList<Integer> list2) {
        LinkedList<Integer> list3 = new LinkedList<Integer>();
        int sum = 0;
        while (!list1.isEmpty() || !list2.isEmpty() || sum != 0) {
            int tempsum = sum;
            if (!list1.isEmpty()) {
                tempsum += list1.poll();
            }
            if (!list2.isEmpty()) {
                tempsum += list2.poll();
            }
            list3.addFirst(tempsum % 10);
            sum = tempsum / 10;
        }
        return list3;
    }

    private static LinkedList<Integer> addForward(LinkedList<Integer> list1,
            LinkedList<Integer> list2) {
        LinkedList<Integer> list3 = new LinkedList<Integer>();
        int sum = 0;
        java.util.Collections.reverse(list1);
        java.util.Collections.reverse(list2);
        while (!list1.isEmpty() || !list2.isEmpty() || sum != 0) {
            int tempsum = sum;
            if (!list1.isEmpty()) {
                tempsum += list1.poll();
            }
            if (!list2.isEmpty()) {
                tempsum += list2.poll();
            }
            list3.addFirst(tempsum % 10);
            sum = tempsum / 10;
        }
        return list3;
    }
}