c++ - 修改后的动态背包 - 有问题的输入？

Question

以下是对 this 的尝试解决方案SPOJ 问题。输入是:

1. 一定数量硬币的总重量
2. 使用货币的硬币的值(value)和相应的权重，目标是找到给定金额的最小可能货币值(value)。

我对 wikipedia article 中背包问题的动态规划解决方案进行了轻微修改关于背包问题——我只是首先按重量对硬币进行排序，这样我就不必遍历所有硬币来获得最小值，并且(希望)确保组合重量等于容量。 (请看代码，它真的很简单并且有注释。)

但是，根据法官的说法，我的算法给出了错误答案的输入。您能指出算法有什么问题吗？

#include <iostream>
#include <vector>
#include <algorithm>
#include <limits>

using namespace std;

typedef unsigned int weight_t;
typedef unsigned int value_t;

struct coin {
    weight_t weight;
    value_t value;
};

coin make_coin(weight_t weight, value_t value) {
    coin ret;
    ret.weight = weight;
    ret.value = value;
    return ret;
}

bool compare_by_weight(const coin& coin1, const coin& coin2) {
    return coin1.weight < coin2.weight;
}

int main() {
    unsigned int test_cases;
    cin >> test_cases;
    while(test_cases--) {
        //Initialization
        unsigned int number_of_coins, n = 0;
        weight_t empty_pig, full_pig, coin_weight, coins_weight;
        value_t coin_value, min_value, current_value = 0;
        vector<coin> coins;
        vector<unsigned int> min_value_for_the_weight;

        //Input
        cin >> empty_pig >> full_pig;
        cin >> number_of_coins;
        n = number_of_coins;
        while(n--) {
            cin >> coin_value >> coin_weight;
            coins.push_back(make_coin(coin_weight, coin_value));
        }

        //Input processing
        coins_weight = full_pig - empty_pig;
        sort(coins.begin(), coins.end(), compare_by_weight);
        min_value_for_the_weight.resize(coins_weight+1);
        for(unsigned int i = 0; i < coins_weight; i++) min_value_for_the_weight[i] = 0;

        //For all weights
        for(unsigned int i = 1; i <= coins_weight; i++) {
            //Find the smallest value
            min_value = numeric_limits<value_t>::max();
            for(unsigned int j = 0; j < number_of_coins; j++) {
                //The examined coin weights more or same than examined total weight and we either already have put a coin
                //in, or this is the first one
                if(coins[j].weight <= i && (min_value_for_the_weight[i - coins[j].weight] > 0 || i == coins[j].weight)){
                    current_value = coins[j].value + min_value_for_the_weight[i - coins[j].weight];
                    if(current_value < min_value) min_value = current_value;
                } else break;  // <- this I deleted to get accepted
            }
            if(min_value == numeric_limits<value_t>::max()) min_value = 0;
            min_value_for_the_weight[i] = min_value;
        }

        //If the piggy empty, output zero
        if(!min_value_for_the_weight[coins_weight] && coins_weight != 0)
            cout << "This is impossible." << endl;
        else
            cout << "The minimum amount of money in the piggy-bank is " << min_value_for_the_weight[coins_weight] << "." << endl;
        }
    return 0;
}

Answer 1

案例empty_pig == full_pig是有问题的，因为你忽略了重复逻辑特例 min_value_for_the_weight 的第零个条目.

另一个错误是它只是一个好主意break如果coins[j].weight > i .旧代码可以 break什么时候coin[j].weight <= i并且合取的另一半是假的，即不可能用重量 i - coins[j].weight 来制造硬币.这发生在以下测试用例中。

1
10 13
2
2 2
3 3

我们必须使用重量为 2 或 3 的硬币来制作重量 3。重量 1 被正确地确定为不可能。权重 2 被正确确定为可能。对于权重3，我们尝试权重2的硬币，确定权重1是不可能的，break在尝试重量为 3 的硬币之前。结果是代码错误地报告不可能使权重为 3。