单链表实现大数相加

字节跳动面试题：

两个数为正数。

用链表求和：

现场写的时候出了几个bug实在尴尬。。。

样例一

2 3 4 5
4 5 6
2 8 0 1

样例二

9 9 9
9 9 9
1 9 9 9

#include <iostream>
using namespace std;

struct List{
    int val;
    List * next;
};

List* insert(List *node, int val){
    List *root = node;
    if(node == NULL){
        // cout << "!!" << endl;
        node = new List();
        root = node;
        node->val = val;
        node->next = NULL;
    }else{
        while(node->next != NULL){
            node = node->next;
        }
        List * an = new List();
        an->val = val;
        an->next = node->next;
        node->next = an;
    }
    return root;
}

void print(List *an){
    while(an != NULL){
        cout << an->val<<" ";
        an = an->next;
    }
    cout << endl;
}


List * become(List *node){
    if(node == NULL) return NULL;
    List *pnext = NULL;
    List *parent = NULL;
    while(node != NULL){
        pnext = node->next;
        node->next = parent;
        parent = node;
        node = pnext;
    }
    return parent;
}

List * getsum(List *a, List *b){
    List *an = become(a);
    List *bn = become(b);
    List *cn = new List();
    int flag = 0;
    while(an != NULL && bn!=NULL){
        List *cnt = new List();
        cnt->val= (an->val + bn->val + flag)%10;
        if((an->val + bn->val + flag)>=10)
            flag = 1;
        else
            flag = 0;
        cnt->next = cn->next;
        cn->next = cnt;
        an = an->next;
        bn = bn->next;
    }
    while(an != NULL){
        List *cnt = new List();
        if((an->val + flag)>=10)
            flag = 1;
        else
            flag = 0;
        cnt->val= (an->val + flag)%10;
        cnt->next = cn->next;
        cn->next = cnt;
        an = an->next;
    }
    while(bn != NULL){
        List *cnt = new List();
        if((bn->val + flag)>=10)
            flag = 1;
        else
            flag = 0;
        cnt->val= (bn->val + flag)%10;
        cnt->next = cn->next;
        cn->next = cnt;
        bn = bn->next;
    }
    if(flag == 1){
        List *cnt = new List();
        cnt->val= 1;
        cnt->next = cn->next;
        cn->next = cnt;
    }
    return cn->next;
}


int main() {
    string s;
    getline(cin, s);
    List *a = NULL, *b = NULL;
    int cnt = 0;
    s += ' ';
    for(int i = 0; i < s.length(); i++){
        if(isdigit(s[i])){
            cnt = cnt*10 + s[i]-'0';
        }else{
            a = insert(a, cnt);
            cnt = 0;
        }
    }
    getline(cin, s);
    cnt = 0;
    s += ' ';
    for(int i = 0; i < s.length(); i++){
        if(isdigit(s[i])){
            cnt = cnt*10 + s[i]-'0';
        }else{
            b= insert(b, cnt);
            cnt = 0;
        }
    }
    List * node = getsum(a,b);
    print(node);
    return 0;
}