Get Neighbors
Lesson 7 of 14 in Coddy's Graphs - Data Structures Series #9 course.
Algorithms like BFS, DFS, and shortest path do not look at the raw map: they ask "what are this vertex's neighbors?" and walk the answer. Our getNeighbors method exposes that.
If the vertex is in the graph, return its neighbor list. Return a copy if you want callers to mutate the result without breaking the graph (this is a nice habit). If the vertex does not exist, return an empty list rather than crashing. That keeps client code simple: it can always iterate the result.
Challenge
EasyAdd a method getNeighbors to the Graph class.
It takes an integer key and returns a list of its neighbors:
- If
keyis invertices, return its neighbor list (a copy is fine). - If
keyis not in the graph, return an empty list.
Try it yourself
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "graph.h"
static int _cmp_int(const void* a, const void* b) {
int ai = *(const int*)a, bi = *(const int*)b;
return (ai > bi) - (ai < bi);
}
int main() {
Graph g;
Graph_init(&g);
char line[1024];
while (fgets(line, sizeof(line), stdin)) {
line[strcspn(line, "\r\n")] = '\0';
char* cmd = strtok(line, " \t");
if (!cmd) continue;
if (strcmp(cmd, "verticesEmpty") == 0) { printf("%s\n", g.vertexCount == 0 ? "true" : "false"); }
if (strcmp(cmd, "hasVertex") == 0) { char* arg = strtok(NULL, " \t"); if (arg) printf("%s\n", Graph_indexOf(&g, atoi(arg)) != -1 ? "true" : "false"); }
if (strcmp(cmd, "addVertex") == 0) { char* arg = strtok(NULL, " \t"); if (arg) Graph_addVertex(&g, atoi(arg)); }
if (strcmp(cmd, "addEdge") == 0) { char* a1 = strtok(NULL, " \t"); char* a2 = strtok(NULL, " \t"); if (a1 && a2) Graph_addEdge(&g, atoi(a1), atoi(a2)); }
if (strcmp(cmd, "degree") == 0) {
char* arg = strtok(NULL, " \t");
if (arg) {
int k = atoi(arg);
int ki = Graph_indexOf(&g, k);
printf("%d\n", ki == -1 ? 0 : g.vertices[ki].n);
}
}
if (strcmp(cmd, "hasEdge") == 0) { char* a1 = strtok(NULL, " \t"); char* a2 = strtok(NULL, " \t"); if (a1 && a2) printf("%s\n", Graph_hasEdge(&g, atoi(a1), atoi(a2)) ? "true" : "false"); }
if (strcmp(cmd, "neighbors") == 0) {
char* arg = strtok(NULL, " \t");
if (arg) {
int buf[MAX_NEIGHBORS];
int n = Graph_getNeighbors(&g, atoi(arg), buf);
qsort(buf, n, sizeof(int), _cmp_int);
for (int i = 0; i < n; i++) {
if (i > 0) printf(" ");
printf("%d", buf[i]);
}
printf("\n");
}
}
}
return 0;
}