Merge pull request #362 from Abdullah-Shah-26/feature/dijkstra_algorithm

Feature/dijkstra algorithm

Author: Pradeepsingh61

CPP/algorithms/graph_algorithms/dijkstra_algorithm.cpp

@@ -0,0 +1,154 @@
+// Dijkstra’s Algorithm — Single Source Shortest Path
+// Problem
+//   Given a weighted graph with non-negative edge weights,
+//   find the shortest path from a source vertex to all other vertices.
+//
+// Approach
+//   1. Use a priority queue (min-heap) to always expand the next closest vertex.
+//   2. Initialize distances[] with infinity, except source = 0.
+//   3. While queue not empty:
+//        - Extract vertex u with smallest dist[u].
+//        - For each neighbor v of u, relax edge (u,v):
+//          if dist[u] + weight(u,v) < dist[v], update dist[v].
+//
+// Complexity
+//   Time  : O((V + E) log V) — using priority queue
+//   Space : O(V + E) — adjacency list + distance array + heap
+//
+// Input
+//   - Number of vertices (V)
+//   - Number of edges (E)
+//   - Edge list {u, v, w} (u → v with weight w)
+//   - Source vertex (src)
+//
+// Output
+//   - Shortest distance from src to all vertices
+
+#include <iostream>
+#include <vector>
+#include <queue>
+#include <limits>
+using namespace std;
+
+int main()
+{
+  ios::sync_with_stdio(false);
+  cin.tie(nullptr);
+
+  int V, E;
+  cin >> V >> E;
+  vector<vector<pair<int, int>>> adj(V);
+
+  for (int i = 0; i < E; i++)
+  {
+    int u, v, w;
+    cin >> u >> v >> w;
+    adj[u].push_back({v, w});
+    adj[v].push_back({u, w}); // undirected; remove if directed
+  }
+
+  int src;
+  cin >> src;
+
+  const int INF = numeric_limits<int>::max();
+  vector<int> dist(V, INF);
+  dist[src] = 0;
+
+  // min-heap {distance, vertex}
+  priority_queue<pair<int, int>, vector<pair<int, int>>, greater<>> pq;
+  pq.push({0, src});
+
+  while (!pq.empty())
+  {
+    int d = pq.top().first;
+    int u = pq.top().second;
+    pq.pop();
+    if (d > dist[u])
+      continue; // outdated entry
+
+    for (auto &edge : adj[u])
+    {
+      int v = edge.first, w = edge.second;
+      if (dist[u] + w < dist[v])
+      {
+        dist[v] = dist[u] + w;
+        pq.push({dist[v], v});
+      }
+    }
+  }
+
+  cout << "Shortest distances from source " << src << ":\n";
+  for (int i = 0; i < V; i++)
+  {
+    if (dist[i] == INF)
+      cout << i << " : INF\n";
+    else
+      cout << i << " : " << dist[i] << "\n";
+  }
+
+  return 0;
+}
+
+/*
+Example Input:
+5 6
+0 1 2
+0 2 4
+1 2 1
+1 3 7
+2 4 3
+3 4 1
+0
+
+Visualization:
+Graph:
+       (2)
+   0 ------- 1
+    \       / \
+   (4)\   (1)  (7)
+      \   /     \
+        2 ------- 3
+         \       /
+         (3)   (1)
+           \   /
+             4
+
+Execution:
+dist = [0, INF, INF, INF, INF]
+Start from src=0
+
+Step 1: Pick 0 → dist[0]=0
+  Relax edges:
+    0→1 (2) → dist[1]=2
+    0→2 (4) → dist[2]=4
+dist = [0,2,4,INF,INF]
+
+Step 2: Pick 1 (dist=2)
+  Relax edges:
+    1→2 (1) → dist[2]=min(4,2+1)=3
+    1→3 (7) → dist[3]=2+7=9
+dist = [0,2,3,9,INF]
+
+Step 3: Pick 2 (dist=3)
+  Relax edges:
+    2→4 (3) → dist[4]=3+3=6
+dist = [0,2,3,9,6]
+
+Step 4: Pick 4 (dist=6) → relax 4→3 (1):
+    dist[3]=min(9,6+1)=7
+dist = [0,2,3,7,6]
+
+Step 5: Pick 3 (dist=7) → no better updates
+
+Final distances from 0:
+0 : 0
+1 : 2
+2 : 3
+3 : 7
+4 : 6
+
+Time  : O((V+E) log V)
+Space : O(V+E)
+*/
+
+// add: Dijkstra PR