Codeforces 10C Digital Root 規律題

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題目連結：點擊開啟連結

#include<stdio.h>#include<iostream>#include<string.h>#include<set>#include<vector>#include<map>#include<math.h>#include<string>#include<stdlib.h>#include<algorithm>using namespace std;#define N 1000005#define ll __int64ll num[10],n;ll go(ll x){ll ans = x;while(ans>=10){ans = 0;while(x) {ans += x%10;x /= 10;}x = ans;}return ans;}int main(){ll i,j,k;while(~scanf("%I64d",&n)){memset(num, 0, sizeof num);ll ans = 0;for(i=1;i<=n;i++)num[go(i)]++, ans -= n/i;for(j=0;j<10;j++)for(k=0;k<10;k++)ans+=num[k]*num[j]*num[go(k*j)];printf("%I64d\n",ans);}return 0;}/*21 231 2 3*/