链表 Add Two Numbers

思想：

注意细节。

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    string ListToString(ListNode *l) {        if(l == NULL) return "0";        string s;        while(l != NULL) {            s.push_back(l->val+'0');            l = l->next;        }        return s;    }    ListNode* StringToListNode(string s) {        ListNode *l, *head;        for(int i = 0; i < s.size(); i++) {            if(i == 0) {                int val = s[i]-'0';                head = new ListNode(val);                l =head;            }            else {                ListNode *node = new ListNode(s[i]-'0');                head->next = node;                head = head->next;            }        }        head->next = NULL;        return l;    }    //Add Two Numbers    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {        string s1 = ListToString(l1);        string s2 = ListToString(l2);        if(s1 == "0" && s2 == "0") {            ListNode *l = new ListNode(0);            return l;        } else if(s1 == "0") {            return StringToListNode(s2);        } else if(s2 == "0") {            return StringToListNode(s1);        }        string s3;        int carry = 0;        int i;        for(i = 0; s1[i]!='\0' && s2[i]!='\0'; i++) {            int sum = s1[i]-'0' + s2[i]-'0' + carry;            s3.push_back(sum%10 + '0');            carry = sum/10;        }        if(s1[i]=='\0') {            while(s2[i]!='\0') {                s3.push_back(((carry+(s2[i]-'0'))%10) + '0');                carry = (carry+(s2[i]-'0'))/10;                i++;            }        } else if(s2[i]=='\0') {            while(s1[i]!='\0') {                s3.push_back(((carry+(s1[i]-'0'))%10) + '0');                carry = (carry+(s1[i]-'0'))/10;                i++;            }        }        if(carry != 0) {            s3.push_back(carry+'0');        }        return StringToListNode(s3);    }};