用C语言编写：从键盘输入创建一个有向图G，以十字链表形式存储在内存中，试写一个判断该有向图中是否有环路（回路）的算法

好的，我们可以修改上面的代码来实现从键盘输入创建有向图的功能。下面是修改后的代码：

#include <stdio.h>
#include <stdlib.h>

typedef struct Vertex {
    int data;
    struct ArcNode* first_in;
    struct ArcNode* first_out;
} Vertex;

typedef struct ArcNode {
    struct Vertex* tail_vertex;
    struct Vertex* head_vertex;
    struct ArcNode* next_in;
    struct ArcNode* next_out;
} ArcNode;

void createGraph(Vertex** graph, int num_vertices) {
    *graph = (Vertex*)malloc(num_vertices * sizeof(Vertex));
    for (int i = 0; i < num_vertices; i++) {
        (*graph)[i].data = i;
        (*graph)[i].first_in = NULL;
        (*graph)[i].first_out = NULL;
    }
}

void addArc(Vertex* graph, int tail, int head) {
    ArcNode* arc = (ArcNode*)malloc(sizeof(ArcNode));
    arc->tail_vertex = &graph[tail];
    arc->head_vertex = &graph[head];
    arc->next_in = graph[head].first_in;
    graph[head].first_in = arc;
    arc->next_out = graph[tail].first_out;
    graph[tail].first_out = arc;
}

int hasCycle(Vertex* graph, int num_vertices) {
    int* visited = (int*)malloc(num_vertices * sizeof(int));
    for (int i = 0; i < num_vertices; i++) {
        visited[i] = 0;
    }

    for (int i = 0; i < num_vertices; i++) {
        if (!visited[i]) {
            if (hasCycleUtil(graph, visited, i)) {
                free(visited);
                return 1;
            }
        }
    }

    free(visited);
    return 0;
}

int hasCycleUtil(Vertex* graph, int* visited, int vertex) {
    visited[vertex] = 1;

    ArcNode* arc = graph[vertex].first_out;
    while (arc != NULL) {
        int next_vertex = arc->head_vertex->data;
        if (!visited[next_vertex] && hasCycleUtil(graph, visited, next_vertex)) {
            return 1;
        } else if (visited[next_vertex]) {
            return 1;
        }

        arc = arc->next_out;
    }

    visited[vertex] = 0;
    return 0;
}

int main() {
    int num_vertices;
    printf("Enter the number of vertices: ");
    scanf("%d", &num_vertices);

    Vertex* graph;
    createGraph(&graph, num_vertices);

    int num_arcs;
    printf("Enter the number of arcs: ");
    scanf("%d", &num_arcs);

    printf("Enter the arcs (tail vertex and head vertex):\n");
    for (int i = 0; i < num_arcs; i++) {
        int tail, head;
        scanf("%d %d", &tail, &head);
        addArc(graph, tail, head);
    }

    int has_cycle = hasCycle(graph, num_vertices);
    if (has_cycle) {
        printf("The graph has a cycle.\n");
    } else {
        printf("The graph does not have a cycle.\n");
    }

    return 0;
}

这段代码会先要求用户输入顶点的数量，然后根据顶点数量创建一个有向图。接下来，要求用户输入边的数量，并提示用户输入每条边的起点和终点。最后，算法会判断图中是否存在环路，并输出结果。

希望对你有所帮助！如果还有其他问题，请随时提问。