Add: Dijkstra's Algorithm in C

C/algorithms/graph/dijkstra.c

@@ -0,0 +1,193 @@
+/**
+ * Dijkstra's Algorithm (Single-Source Shortest Path) - C Implementation
+ *
+ * Description:
+ * Finds the shortest path distances from a source vertex to all other vertices
+ * in a weighted directed graph with non-negative edge weights.
+ *
+ * Input format (from stdin):
+ *  - First line: V E           (number of vertices and edges)
+ *  - Next E lines: u v w       (edge from u to v with weight w)
+ *  - Last line: src            (source vertex)
+ *  Vertices are 0-indexed in the range [0, V-1].
+ *
+ * Output:
+ *  - Prints one line per vertex: "dist[src->i] = D" where D is the distance
+ *    from src to i, or INF if unreachable.
+ *
+ * Time Complexity:
+ *  - Using adjacency list + simple O(V) min selection: O(V^2 + E)
+ *    (Good for small/medium graphs without extra dependencies.)
+ *  - With a binary heap priority queue, it can be improved to O((V + E) log V).
+ *
+ * Space Complexity:
+ *  - O(V + E) for the adjacency list and auxiliary arrays.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <limits.h>
+
+typedef struct Edge {
+    int to;
+    int weight;
+    struct Edge *next;
+} Edge;
+
+typedef struct {
+    Edge **heads;
+    int numVertices;
+} Graph;
+
+static Edge *createEdge(int to, int weight) {
+    Edge *edge = (Edge *)malloc(sizeof(Edge));
+    if (!edge) {
+        fprintf(stderr, "Memory allocation failed for Edge.\n");
+        exit(EXIT_FAILURE);
+    }
+    edge->to = to;
+    edge->weight = weight;
+    edge->next = NULL;
+    return edge;
+}
+
+static Graph createGraph(int numVertices) {
+    Graph graph;
+    graph.numVertices = numVertices;
+    graph.heads = (Edge **)calloc(numVertices, sizeof(Edge *));
+    if (!graph.heads) {
+        fprintf(stderr, "Memory allocation failed for graph heads.\n");
+        exit(EXIT_FAILURE);
+    }
+    return graph;
+}
+
+static void addEdge(Graph *graph, int from, int to, int weight) {
+    Edge *edge = createEdge(to, weight);
+    edge->next = graph->heads[from];
+    graph->heads[from] = edge;
+}
+
+static void freeGraph(Graph *graph) {
+    for (int i = 0; i < graph->numVertices; i++) {
+        Edge *curr = graph->heads[i];
+        while (curr) {
+            Edge *next = curr->next;
+            free(curr);
+            curr = next;
+        }
+    }
+    free(graph->heads);
+}
+
+static int extractMinUnvisited(int *dist, int *visited, int V) {
+    int minIndex = -1;
+    int minValue = INT_MAX;
+    for (int i = 0; i < V; i++) {
+        if (!visited[i] && dist[i] < minValue) {
+            minValue = dist[i];
+            minIndex = i;
+        }
+    }
+    return minIndex;
+}
+
+static void dijkstra(const Graph *graph, int source, int *dist) {
+    int V = graph->numVertices;
+    int *visited = (int *)calloc(V, sizeof(int));
+    if (!visited) {
+        fprintf(stderr, "Memory allocation failed for visited array.\n");
+        exit(EXIT_FAILURE);
+    }
+
+    for (int i = 0; i < V; i++) {
+        dist[i] = INT_MAX;
+    }
+    dist[source] = 0;
+
+    for (int count = 0; count < V - 1; count++) {
+        int u = extractMinUnvisited(dist, visited, V);
+        if (u == -1) {
+            break; // No reachable unvisited vertices remain
+        }
+        visited[u] = 1;
+
+        for (Edge *edge = graph->heads[u]; edge != NULL; edge = edge->next) {
+            int v = edge->to;
+            int w = edge->weight;
+            if (!visited[v] && dist[u] != INT_MAX && dist[u] + w < dist[v]) {
+                dist[v] = dist[u] + w;
+            }
+        }
+    }
+
+    free(visited);
+}
+
+static void printDistances(const int *dist, int V, int src) {
+    for (int i = 0; i < V; i++) {
+        if (dist[i] == INT_MAX) {
+            printf("dist[%d->%d] = INF\n", src, i);
+        } else {
+            printf("dist[%d->%d] = %d\n", src, i, dist[i]);
+        }
+    }
+}
+
+int main() {
+    int V, E;
+    if (scanf("%d %d", &V, &E) != 2) {
+        fprintf(stderr, "Invalid input. Expected: V E\n");
+        return 1;
+    }
+
+    if (V <= 0) {
+        fprintf(stderr, "Number of vertices must be positive.\n");
+        return 1;
+    }
+
+    Graph graph = createGraph(V);
+
+    for (int i = 0; i < E; i++) {
+        int u, v, w;
+        if (scanf("%d %d %d", &u, &v, &w) != 3) {
+            fprintf(stderr, "Invalid edge input at line %d. Expected: u v w\n", i + 1);
+            freeGraph(&graph);
+            return 1;
+        }
+        if (u < 0 || u >= V || v < 0 || v >= V || w < 0) {
+            fprintf(stderr, "Invalid edge values: u=%d v=%d w=%d\n", u, v, w);
+            freeGraph(&graph);
+            return 1;
+        }
+        addEdge(&graph, u, v, w);
+    }
+
+    int src;
+    if (scanf("%d", &src) != 1) {
+        fprintf(stderr, "Invalid input. Expected source vertex.\n");
+        freeGraph(&graph);
+        return 1;
+    }
+    if (src < 0 || src >= V) {
+        fprintf(stderr, "Source vertex out of range.\n");
+        freeGraph(&graph);
+        return 1;
+    }
+
+    int *dist = (int *)malloc(V * sizeof(int));
+    if (!dist) {
+        fprintf(stderr, "Memory allocation failed for dist array.\n");
+        freeGraph(&graph);
+        return 1;
+    }
+
+    dijkstra(&graph, src, dist);
+    printDistances(dist, V, src);
+
+    free(dist);
+    freeGraph(&graph);
+    return 0;
+}
+
+