Problem C. Concatenation

AC代码（让数组a和数组b的长度相乘然后减去a中a和b想同的字母的个数注意用long long）

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#include <bits/stdc++.h>
#include <iostream>
using namespace std;

int main()
{
    freopen("concatenation.in", "r", stdin);
    freopen("concatenation.out","w",stdout);
    char a[100000+10], b[100000+10];
    int  n, m, i,  sum[100000+10];
    long long ans = 0;
    cin>>a>>b;
    n = strlen(a), m = strlen(b);
    ans = (long long)n*(long long)m;
    for(i = 1;i<n;i++)
    {
        sum[a[i]]++;
    }
    for(i = 0;i<m-1;i++)
    {
        ans-=sum[b[i]];
    }
    printf("%lld\n",ans);
    return 0;
}