算法提高 快乐司机

/**
    2017.3.25
    Donald
*/


/**
    思路：不用dp，
    直接按其价值比，从高到底堆积。
*/
#include<bits/stdc++.h>

using namespace std;

#define MAXN 10001

int N;  //物品的数目
int W;  //荷载重量

//pair.second .first : weight, .second : value;
pair<double , pair<int, int> > Pair[MAXN];

void solve() {

    double value = 0;
    int remainWeight = W;

    sort(Pair, Pair + N);

    for (int i = N - 1; i >= 0; i --) {
        if (remainWeight <= 0)
            break;
        if (Pair[i].second.first >= remainWeight) {
            value += remainWeight * Pair[i].first;
            remainWeight = 0;
//            printf("value: %d , weight : %d\n", Pair[i].second.first, Pair[i].second.second);
        } else {
            value += Pair[i].second.second;
            remainWeight -= Pair[i].second.first;
//            printf("value: %d , weight : %d\n", Pair[i].second.first, Pair[i].second.second);
        }
    }
    printf("%.1lf", value);
}

int main(void) {

    int gi, pi;
    double rate;
    scanf("%d%d", &N, &W);
    for (int i = 0 ; i < N; ++i) {
        scanf("%d%d", &gi, &pi);
        rate = pi * 1.0 / gi;
        Pair[i].first = rate;
        Pair[i].second.first = gi;
        Pair[i].second.second = pi;
    }

    solve();

    return 0;
}