c++ - Kruskal 的算法实现

Question

我打算用 C++ 实现 Kriskal 的算法，但是...

Unhandled exception at 0x0127160d in DAA.exe: 0xC0000005: Access violation reading location 0xdd2021d4.

它在 getRoot 函数的这一行停止:

while(cities[root].prev != NO_PARENT)

我认为问题出在城市数组中的数据。当我打印数组中的所有数据时，这不是我想要的。城市的名称是这样的“════════════════¤¤¤¤ллллллллю■ю■”和数字(int) - 像这样(-842150451)。以下是完整代码。

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>

#define NO_PARENT -1

struct city {
    char name[11];
    int prev;
};

struct path {
    unsigned i, j, price;
};

bool comparsion(path p1, path p2) {
    return p1.price > p2.price;
}

int getRoot(city *cities, int cityNumber) {
    int root = cityNumber, tmp;

    while(cities[root].prev != NO_PARENT)
        root = cities[root].prev;

    while(cityNumber != root) {
        tmp = cityNumber;
        cityNumber = cities[cityNumber].prev;
        cities[tmp].prev = root;
    }

    return root;
}

bool isListed(city *cities, int n, char cityName[]) {
    for(int i = 0; i < n; i++)
        if(strcmp(cities[i].name, cityName))
            return true;
    return false;
}

int getCityNumber(city *cities, int n, char cityName[]) {
    for(int i = 0; i < n; i++)
        if(strcmp(cities[i].name, cityName))
            return i;
    return NO_PARENT;
}

int minPrice(city *cities, path *paths, int cityCount, int pathCount) {
    unsigned minPrice = 0;
    // sort paths by price
    std::sort(paths, &paths[pathCount-1], comparsion);

    for(int k = 0; k < pathCount; k++) {
        printf("path: %d - %d\n", paths[k].i, paths[k].j);
        int c1 = getRoot(cities, paths[k].i), c2 = getRoot(cities, paths[k].j);
        if(c1 != c2) {
            minPrice += paths[k].price;
            cities[c2].prev = c1;
        }
    }

    return minPrice;
}

    int main() {
    int n, m, k;
    do {
        scanf("%d %d %d", &n, &m, &k);
    } while(n < 2 || n > 10001 || m < -1 || m > 100001 || k < -1 || k > 100001);

    city* cities = (city*)malloc(n*sizeof(city));
    path* paths = (path*)malloc((m + k)*sizeof(path));
    int addCities = 0;
    char city1[11], city2[11];
    for(int i = 0; i < (m + k); i++) {
        scanf("%s %s", city1, city2);

        if(addCities < n && !isListed(cities, n, city1)) { // if city1 is not into cities
            // add it
            strcpy(cities[addCities].name, city1);
            cities[addCities].prev = NO_PARENT;
            addCities++;
        }
        paths[i].i = getCityNumber(cities, n, city1); // number of city1

        if(addCities < n && !isListed(cities, n, city2)) { // if city2 is not into cities
            // add it
            strcpy(cities[addCities].name, city2);
            cities[addCities].prev = NO_PARENT;
            addCities++;
        }
        paths[i].j = getCityNumber(cities, n, city1); // number of city2

        if(i >= m)
            scanf("%d", &paths[i].price);
    }

    for(int i = 0; i < (m + k); i++)
        printf("%s: %d\n", cities[i].name, cities[i].prev);

    // Calculate min price
    printf("%d ", minPrice(cities, paths, n, k + m));

    system("pause");
    return 0;
}

Answer 1

在 isListed() 和 getCityNumber() 中，您使用 strcmp() 来检查字符串是否相等。你这样做的方式有两个问题:

1. 当两个字符串相等时，strcmp 返回 0，因此您需要检查 if( strcmp(...) == 0 )。这是 C 中的这些奇怪的事情之一。

2. 在 malloc'ing 之后，您需要将 cities[i].name 设置为某物，例如“未命名”或只是“\0”。否则，将在未初始化的字符串上调用 strcmp - 如果它们不包含 11 个字符内的空字符，它将失败。在 malloc 行之后添加此代码:

   for( int i = 0 ; i < n ; ++ i ) {
       cities[ i ].name[ 0 ] = '\0';
       cities[ i ].parent    = NO_PARENT;
   }