From 41efb7277792e9d1502d305cfd706dd514e24663 Mon Sep 17 00:00:00 2001
From: Haidong Ji
Date: Mon, 5 Jul 2021 14:38:04 -0500
Subject: Dijkstra shortest path done, using PriorityQueue. Don't give up! Very
 rewarding!

---
 src/main/Dijkstra.java   | 64 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/main/PlayGround.java | 13 ++++++++++
 2 files changed, 77 insertions(+)
 create mode 100644 src/main/Dijkstra.java
 create mode 100644 src/main/PlayGround.java

(limited to 'src/main')

diff --git a/src/main/Dijkstra.java b/src/main/Dijkstra.java
new file mode 100644
index 0000000..5bc31b0
--- /dev/null
+++ b/src/main/Dijkstra.java
@@ -0,0 +1,64 @@
+import java.util.*;
+
+public class Dijkstra {
+    static int distance(ArrayList<Integer>[] adj, ArrayList<Integer>[] cost, int s, int t) {
+        if (s == t)
+            return 0;
+        int[] dist = new int[adj.length];
+        //int[] prev = new int[adj.length];
+        boolean[] visited = new boolean[adj.length];
+        for (int i = 0; i < dist.length; i++) {
+            dist[i] = Integer.MAX_VALUE;
+            //prev[i] = -1;
+        }
+
+        dist[s] = 0;
+
+        PriorityQueue<Integer> vertexQ;
+        vertexQ = new PriorityQueue<Integer>(Comparator.comparingInt(a -> dist[a]));
+        vertexQ.add(s);
+
+        while (!vertexQ.isEmpty()) {
+            int u = vertexQ.remove();
+            if (visited[u]) {
+                continue;
+            }
+            visited[u] = true;
+            if (adj[u].size() > 0) {
+                for (int i = 0; i < adj[u].size(); i++) {
+                    if (dist[adj[u].get(i)] > dist[u] + cost[u].get(i)) {
+                        dist[adj[u].get(i)] = dist[u] + cost[u].get(i);
+                        //prev[adj[u].get(i)] = u;
+                        vertexQ.add(adj[u].get(i));
+                    }
+                }
+            }
+        }
+        if (dist[t] == Integer.MAX_VALUE)
+            return -1;
+        return dist[t];
+    }
+
+    public static void main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+        int n = scanner.nextInt();
+        int m = scanner.nextInt();
+        ArrayList<Integer>[] adj = (ArrayList<Integer>[]) new ArrayList[n];
+        ArrayList<Integer>[] cost = (ArrayList<Integer>[]) new ArrayList[n];
+        for (int i = 0; i < n; i++) {
+            adj[i] = new ArrayList<Integer>();
+            cost[i] = new ArrayList<Integer>();
+        }
+        for (int i = 0; i < m; i++) {
+            int x, y, w;
+            x = scanner.nextInt();
+            y = scanner.nextInt();
+            w = scanner.nextInt();
+            adj[x - 1].add(y - 1);
+            cost[x - 1].add(w);
+        }
+        int x = scanner.nextInt() - 1;
+        int y = scanner.nextInt() - 1;
+        System.out.println(distance(adj, cost, x, y));
+    }
+}
\ No newline at end of file
diff --git a/src/main/PlayGround.java b/src/main/PlayGround.java
new file mode 100644
index 0000000..14a4c80
--- /dev/null
+++ b/src/main/PlayGround.java
@@ -0,0 +1,13 @@
+import java.util.PriorityQueue;
+import java.util.HashMap;
+public class PlayGround {
+    static int testPriorityQueue() {
+        PriorityQueue<Integer> numbers = new PriorityQueue<>();
+        numbers.add(7);
+        numbers.add(5);
+        numbers.add(8);
+        numbers.add(3);
+       return numbers.poll();
+    }
+
+}
-- 
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