UVaLive 7500 Boxes and Balls （数学）

#include <cstdio>
#include <string>
#include <cstdlib>
#include <cmath>
#include <iostream>
#include <cstring>
#include <set>
#include <queue>
#include <algorithm>
#include <vector>
#include <map>
#include <cctype>
using namespace std ;
typedef unsigned long long LL;
typedef pair<int, int> P;
const int INF = 0x3f3f3f3f;
const double inf = 0x3f3f3f3f3f3f3f;
const double eps = 1e-8;
const int maxn = 1e6 + 5;
const int dr[] = {0, 0, -1, 1};
const int dc[] = {-1, 1, 0, 0};

int main(){
    int T;  cin >> T;
    LL x;
    for(int kase = 1; kase <= T; ++kase){
        cin >> x;
        LL y = (LL)sqrt(2.0*x*1.0);
        LL ans = y*(y-1)/2;
        for(LL i = y-1; ; ++i){
            if(i * (i+1) <= 2*x)  ans = max(ans, i*(i+1)/2);
            else break;
        }
        printf("Case #%d: ", kase);
        cout << ans << endl;
    }
    return 0;
}