Codeforces--424B--Megacity

题目描述：
The administration of the Tomsk Region firmly believes that it’s time to become a megacity (that is, get population of one million). Instead of improving the demographic situation, they decided to achieve its goal by expanding the boundaries of the city.
The city of Tomsk can be represented as point on the plane with coordinates (0; 0). The city is surrounded with n other locations, the i-th one has coordinates (xi, yi) with the population of ki people. You can widen the city boundaries to a circle of radius r. In such case all locations inside the circle and on its border are included into the city.
Your goal is to write a program that will determine the minimum radius r, to which is necessary to expand the boundaries of Tomsk, so that it becomes a megacity.
输入描述:
The first line of the input contains two integers n and s (1 ≤ n ≤ 103; 1 ≤ s < 106) — the number of locatons around Tomsk city and the population of the city. Then n lines follow. The i-th line contains three integers — the xi and yi coordinate values of the i-th location and the number ki of people in it (1 ≤ ki < 106). Each coordinate is an integer and doesn’t exceed 104 in its absolute value.
It is guaranteed that no two locations are at the same point and no location is at point (0; 0).
输出描述:
In the output, print “-1” (without the quotes), if Tomsk won’t be able to become a megacity. Otherwise, in the first line print a single real number — the minimum radius of the circle that the city needs to expand to in order to become a megacity.
The answer is considered correct if the absolute or relative error don’t exceed 10 - 6.
输入：
4 999998
1 1 1
2 2 1
3 3 1
2 -2 1
4 999998
1 1 2
2 2 1
3 3 1
2 -2 1
2 1
1 1 999997
2 2 1
输出：
2.8284271
1.4142136
-1
题意：
给出城市（0,0），给出n个坐标，起始人数s,每个坐标k个人，每个坐标可以覆盖到半径为r的区域，r=sqrt(xx+yy)的区域，问最小的半径是多少，使得城市的总人数大于等于1000000
题解：
排序，贪心来做
代码：

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<cmath>
using namespace std;const int maxn = 10000 + 5;
struct point{int k;double r;
};
int n,s;
point a[maxn];int cmp(point x,point y){return x.r < y.r;
}int main(){int x,y;while(scanf("%d%d",&n,&s)!=EOF){int sum = s;for(int i = 0; i < n; i ++){scanf("%d%d%d",&x,&y,&a[i].k);a[i].r = sqrt(x * x + y * y);}bool flag = false;double ans = 0;sort(a,a + n,cmp);for(int i = 0; i < n; i ++){sum += a[i].k;if(sum >= 1000000){flag = true;ans = a[i].r;break;}}if(!flag) printf("-1\n");else{printf("%.7lf\n",ans);}}return 0;
}

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