LightOJ1245 Harmonic Number (II)
题意
\(求\Sigma \lfloor \frac{n}{i} \rfloor\)
Input starts with an integer T (≤ 1000), denoting the number of test cases.
Each case starts with a line containing an integer n (1 ≤ n < 2^31).
Sol
数论分块
# include <bits/stdc++.h>
# define RG register
# define IL inline
# define Fill(a, b) memset(a, b, sizeof(a))
using namespace std;
typedef long long ll;
IL ll Read(){
char c = '%'; ll x = 0, z = 1;
for(; c > '9' || c < '0'; c = getchar()) if(c == '-') z = -1;
for(; c >= '0' && c <= '9'; c = getchar()) x = x * 10 + c - '0';
return x * z;
}
ll n, ans;
int main(RG int argc, RG char *argv[]){
for(RG int T = Read(), Case = 1; Case <= T; ++Case){
n = Read(); ans = 0;
for(RG ll i = 1, j; i <= n; i = j + 1){
j = min(n, n / (n / i));
ans += 1LL * (n / i) * (j - i + 1);
}
printf("Case %d: %lld\n", Case, ans);
}
return 0;
}