Submission

Score: 90

User: Quinruj

Problemset: เลขหลักของผลคูณ

Language: cpp

Time: 0.002 second

Submitted On: 2025-10-05 20:49:51

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

string pls(string a,string b){
    reverse(a.begin(),a.end());
    reverse(b.begin(),b.end());
    while (a.size() < b.size()) a += '0';
    while (b.size() < a.size()) b += '0';
    reverse(a.begin(),a.end());
    reverse(b.begin(),b.end());
    string c = "";
    int tod = 0;
    for (int i = a.size()-1;i>=0;i--){
        int sum = (a[i]-'0')+(b[i]-'0')+tod;
        c += ((sum%10)+'0');
        tod = sum/10;
    }
    if (tod != 0) c+=(tod+'0');
    reverse(c.begin(),c.end());
    return c;
}

string mul(string a,string b){
    if (a.size() < b.size()) swap(a,b);
    vector<string> w;
    for (int i = b.size()-1;i>=0;i--){
        string c = "";
        int tod = 0;
        for (int j = a.size()-1;j>=0;j--){
            int multi = (b[i]-'0')*(a[j]-'0')+tod;
            c += ((multi%10)+'0');
            tod = multi/10;
        }
        if (tod != 0) c += (tod+'0');
        reverse(c.begin(),c.end());
        int cnt = b.size()-i-1;
        for (int i = 0;i<cnt;i++) c+='0';
        w.push_back(c);
    }
    string sum = "0";
    for (int i = 0;i<w.size();i++){
        sum = pls(w[i],sum);
    }
    return sum;
}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    
    string a,b;cin>>a>>b;
    int x;cin>>x;
    string s = mul(a,b);
    if (x < s.size() && x > 0) cout << s[x-1];
    else cout << '_';
}