5.11

終於開始做中等難度的題了，也算一個小小的里程碑了吧。

這個題，我是把連結串列轉換成了字串來做的。本來想轉換成整型，但是想一定會越界的，最後還是採用了字串。

就是最後的最後忘記了最高位的進位。

這個題自己感覺還算比較簡單的吧。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;      
 *     }
 * }
 */
public class Solution {
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2 
     */
    public ListNode addLists2(ListNode l1, ListNode l2) {
        // write your code here
        if(l1 == null){
            return l2;
        }
        if(l2 == null){
            return l1;
        }
        String s1 = "";
        String s2 = "";
        while(l1 != null){
            s1 = s1+ Integer.toString(l1.val);
            l1 = l1.next;
        }
        while(l2 != null){
            s2 = s2 + Integer.toString(l2.val);
            l2 = l2.next;
        }
        int length1 = s1.length() -1;
        int length2 = s2.length() -1;
        int flag = 0;
        ListNode head = new ListNode(-1);
        ListNode tmp = head;
        while(length1 >= 0 && length2 >= 0){
            int t = s1.charAt(length1) + s2.charAt(length2) -'0'-'0' + flag;
            flag = t/10;
            t = t%10;
            ListNode newNode = new ListNode(t);
            newNode.next = head.next;
            head.next = newNode;
            length1 --;
            length2 --;
            //System.out.println("t：" + t);
        }
        while(length1 >=0 ){
            int t = s1.charAt(length1) -'0' + flag;
            flag = t/10;
            t = t%10;
            ListNode newNode = new ListNode(t);
            newNode.next = head.next;
            head.next = newNode;
            length1 --;
        }
        while(length2 >=0 ){
            int t = s2.charAt(length2) -'0' + flag;
            flag = t/10;
            t = t%10;
            ListNode newNode = new ListNode(t);
            newNode.next = head.next;
            head.next = newNode;
            length2 --;
        }
        if(flag == 1){
            ListNode newNode = new ListNode(1);
            newNode.next = head.next;
            head.next = newNode;
        }
        return head.next;
    }  
}