CodeForces 625D Finals in arithmetic

#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <queue>
#include <stack>
#include <map>
#include <vector>
using namespace std;

const int maxn = 100000 + 100;
char s[maxn];
int a[maxn], b[maxn], sum[maxn], flag[maxn];
int tmpa[maxn], tmpb[maxn], ans[maxn];
int nb[maxn];
int len;


void read()
{
    scanf("%s", s);
    for (int i = 0; s[i]; i++) b[i + 1] = s[i] - ‘0‘;
    nb[1] = b[1] * 10 + b[2]; for (int i = 2; s[i]; i++) nb[i] = s[i] - ‘0‘;
}

void init()
{
    len = strlen(s);
    memset(flag, 0, sizeof flag);
}

bool check1()
{
    for (int i = 1; i <= (len + 1) / 2; i++)
    {
        if (sum[i] % 2 == 0) a[i] = sum[i] / 2, a[len - i + 1] = sum[i] / 2;
        else a[i] = sum[i] / 2 + 1, a[len - i + 1] = sum[i] - a[i];
    }

    for (int i = 1; i <= len; i++) tmpa[i] = a[i];
    for (int i = 1; i <= len; i++) tmpb[i] = a[len - i + 1];

    int k = 0;
    for (int i = 1; i <= len; i++)
    {
        ans[i] = (tmpa[i] + tmpb[i] + k) % 10;
        k = (tmpa[i] + tmpb[i] + k) / 10;
    }

    for (int i = 1; i <= len; i++)
    if (b[len - i + 1] != ans[i]) return 0;
    return 1;
}

bool work1()
{
    bool fail = 0; init();

    for (int i = len; i >= len / 2 + 1; i--)
    {
        int id_now = i, id_pre = len - i + 1;

        if (i == len)
        {
            if (b[i] == 0) { fail = 1; break; }

            sum[id_now] = b[id_now]; sum[id_pre] = b[id_now];
            flag[id_now] = 0; flag[id_pre] = 0;

            if (b[id_now] == b[id_pre]) flag[id_pre + 1] = 0;
            else if (b[id_now] + 1 == b[id_pre]) flag[id_pre + 1] = 1;
            else { fail = 1; break; }
        }

        else
        {
            int num_now, num_pre;
            num_now = b[id_now] - flag[id_now + 1];

            if (num_now == 9)
            {
                if (b[id_pre] == 9)
                {
                    sum[id_now] = 9; sum[id_pre] = 9;
                    flag[id_pre + 1] = 0;
                }
                else if (b[id_pre] == 0)
                {
                    sum[id_now] = 9; sum[id_pre] = 9;
                    flag[id_pre + 1] = 1;
                }
                else { fail = 1; break; }
            }
            else if (b[id_now] == 0 && flag[id_now + 1])
            {
                if (b[id_pre] == 9 || b[id_pre] == 0)
                {
                    flag[id_now] = 1;
                    if (b[id_pre] == 0) flag[id_pre + 1] = 1;
                    sum[id_now] = 9;
                    sum[id_pre] = 9;
                }
                else { fail = 1; break; }
            }
            else
            {
                num_now = num_now + flag[id_pre] * 10;
                num_pre = b[id_pre] + flag[id_pre] * 10;

                if (num_now == num_pre)
                {
                    flag[id_pre + 1] = 0;
                    sum[id_now] = num_now; sum[id_pre] = num_now;
                    if (num_now >= 10) flag[id_now] = 1;
                }

                else if (num_now + 1 == num_pre)
                {
                    flag[id_pre + 1] = 1;
                    sum[id_now] = num_now; sum[id_pre] = num_now;
                    if (num_now >= 10) flag[id_now] = 1;
                }
                else { fail = 1; break; }
            }
        }
        if (id_now == id_pre&&sum[id_pre] % 2 != 0) { fail = 1; break; }
    }

    if (!check1()) fail = 1;

    return fail;
}

bool check2()
{
    memset(tmpa, 0, sizeof tmpa);
    memset(tmpb, 0, sizeof tmpb);

    for (int i = 1; i <= (len + 1) / 2; i++)
    {
        if (sum[i] % 2 == 0) a[i] = sum[i] / 2, a[len - i + 1] = sum[i] / 2;
        else a[i] = sum[i] / 2 + 1, a[len - i + 1] = sum[i] - a[i];
    }

    for (int i = 1; i <= len; i++) tmpa[i] = a[i];
    for (int i = 1; i <= len; i++) tmpb[i] = a[len - i + 1];

    int k = 0;
    len++;
    for (int i = 1; i <= len; i++)
    {
        ans[i] = (tmpa[i] + tmpb[i] + k) % 10;
        k = (tmpa[i] + tmpb[i] + k) / 10;
    }

    for (int i = 1; i <= len; i++)
    if (b[len - i + 1] != ans[i]) return 0;
    return 1;
}

bool work2()
{
    bool fail = 0; init();

    len--;

    for (int i = len; i >= len / 2 + 1; i--)
    {
        int id_now = i, id_pre = len - i + 1;

        if (i == len)
        {
            sum[id_now] = 10 + nb[id_now]; sum[id_pre] = 10 + nb[id_now];
            flag[id_now] = 1; flag[id_pre] = 1;

            if (nb[id_now] + 10 == nb[id_pre]) flag[id_pre + 1] = 0;
            else if (nb[id_now] + 10 + 1 == nb[id_pre]) flag[id_pre + 1] = 1;
            else { fail = 1; break; }
        }

        else
        {
            int num_now, num_pre;
            num_now = nb[id_now] - flag[id_now + 1];

            if (num_now == 9)
            {
                if (nb[id_pre] == 9)
                {
                    sum[id_now] = 9; sum[id_pre] = 9;
                    flag[id_pre + 1] = 0;
                }
                else if (nb[id_pre] == 0)
                {
                    sum[id_now] = 9; sum[id_pre] = 9;
                    flag[id_pre + 1] = 1;
                }
                else { fail = 1; break; }
            }
            else if (nb[id_now] == 0 && flag[id_now + 1])
            {
                if (nb[id_pre] == 9 || nb[id_pre] == 0)
                {
                    flag[id_now] = 1;
                    if (nb[id_pre] == 0) flag[id_pre + 1] = 1;
                    sum[id_now] = 9;
                    sum[id_pre] = 9;
                }
                else { fail = 1; break; }
            }
            else
            {
                num_now = num_now + flag[id_pre] * 10;
                num_pre = nb[id_pre] + flag[id_pre] * 10;

                if (num_now == num_pre)
                {
                    flag[id_pre + 1] = 0;
                    sum[id_now] = num_now; sum[id_pre] = num_now;
                    if (num_now >= 10) flag[id_now] = 1;
                }

                else if (num_now + 1 == num_pre)
                {
                    flag[id_pre + 1] = 1;
                    sum[id_now] = num_now; sum[id_pre] = num_now;
                    if (num_now >= 10) flag[id_now] = 1;
                }
                else { fail = 1; break; }
            }
        }
        if (id_now == id_pre&&sum[id_pre] % 2 != 0) { fail = 1; break; }
    }

    if (!check2()) fail = 1;

    return fail;
}


int main()
{
    read();
    init();
    if (!work1())
    {
        for (int i = 1; i <= len; i++) printf("%d", a[i]);
        printf("\n");
    }
    else
    {
        if (b[1] == 1)
        {
            if (!work2())
            {
                for (int i = 1; i <= len - 1; i++) printf("%d", a[i]);
                printf("\n");
            }
            else { printf("0\n"); }
        }
        else { printf("0\n"); }
    }
    return 0;
}