sp

Author: gdhucoder

Ch_4_4/TestDijkstraSP.java

@@ -0,0 +1,34 @@
+package Ch_4_4;
+
+import edu.princeton.cs.algs4.In;
+import edu.princeton.cs.algs4.StdOut;
+
+/**
+ * Created by HuGuodong on 5/12/20.
+ */
+public class TestDijkstraSP {
+
+  public static void main(String[] args) {
+    _EdgeWeightedDigraph G = new _EdgeWeightedDigraph(new In("algdata/tinyEWD.txt"));
+    int s = 0;
+    _DijkstraSP sp = new _DijkstraSP(G, s);
+    for (int t = 0; t < G.V(); t++) {
+      StdOut.print(s + " to " + t);
+      StdOut.printf(" (%4.2f): ", sp.distTo(t));
+      if (sp.hasPathTo(t)) {
+        for (_DirectedEdge e : sp.pathTo(t)) {
+          StdOut.print(e + " ");
+        }
+      }
+      StdOut.println();
+    }
+    //    0 to 0 (0.00):
+    //    0 to 1 (1.05): 0->4 0.38 4->5 0.35 5->1 0.32
+    //    0 to 2 (0.26): 0->2 0.26
+    //    0 to 3 (0.99): 0->2 0.26 2->7 0.34 7->3 0.39
+    //    0 to 4 (0.38): 0->4 0.38
+    //    0 to 5 (0.73): 0->4 0.38 4->5 0.35
+    //    0 to 6 (1.51): 0->2 0.26 2->7 0.34 7->3 0.39 3->6 0.52
+    //    0 to 7 (0.60): 0->2 0.26 2->7 0.34
+  }
+}

Ch_4_4/_DijkstraSP.java

@@ -0,0 +1,58 @@
+package Ch_4_4;
+
+import edu.princeton.cs.algs4.IndexMinPQ;
+import edu.princeton.cs.algs4.Stack;
+
+/**
+ * Created by HuGuodong on 5/12/20.
+ */
+public class _DijkstraSP {
+
+  private _DirectedEdge[] edgeTo;
+  private double[] distTo;
+  private IndexMinPQ<Double> pq;
+
+  public _DijkstraSP(_EdgeWeightedDigraph G, int s) {
+    edgeTo = new _DirectedEdge[G.V()];
+    distTo = new double[G.V()];
+    pq = new IndexMinPQ<>(G.V());
+    for (int v = 0; v < G.V(); v++) {
+      distTo[v] = Double.POSITIVE_INFINITY;
+    }
+    distTo[s] = 0.0;
+
+    pq.insert(s, 0.0);
+    while (!pq.isEmpty()) {
+      relax(G, pq.delMin());
+    }
+  }
+
+  private void relax(_EdgeWeightedDigraph G, int v) {
+    for (_DirectedEdge e : G.adj(v)) {
+      int w = e.to();
+      if (distTo[w] > distTo[v] + e.weight()) {
+        distTo[w] = distTo[v] + e.weight();
+        edgeTo[w] = e;
+        if (!pq.contains(w)) pq.insert(w, distTo[w]);
+        else pq.changeKey(w, distTo[w]);
+      }
+    }
+  }
+
+  public double distTo(int v) {
+    return distTo[v];
+  }
+
+  public boolean hasPathTo(int v) {
+    return distTo[v] < Double.POSITIVE_INFINITY;
+  }
+
+  public Iterable<_DirectedEdge> pathTo(int v) {
+    if (!hasPathTo(v)) return null;
+    Stack<_DirectedEdge> path = new Stack<>();
+    for (_DirectedEdge e = edgeTo[v]; e != null; e = edgeTo[e.from()]) {
+      path.push(e);
+    }
+    return path;
+  }
+}

Ch_4_4/_EdgeWeightedDigraph.java

@@ -1,6 +1,7 @@
 package Ch_4_4;
 
 import edu.princeton.cs.algs4.Bag;
+import edu.princeton.cs.algs4.In;
 
 /**
  * Created by HuGuodong on 5/12/20.
@@ -20,6 +21,18 @@ public _EdgeWeightedDigraph(int V) {
     }
   }
 
+  public _EdgeWeightedDigraph(In in) {
+    this(in.readInt());
+    int E = in.readInt();
+    for (int i = 0; i < E; i++) {
+      int v = in.readInt();
+      int w = in.readInt();
+      double weight = in.readDouble();
+      _DirectedEdge e = new _DirectedEdge(v, w, weight);
+      addEdge(e);
+    }
+  }
+
   public int V() {
     return this.V;
   }
@@ -36,4 +49,14 @@ public void addEdge(_DirectedEdge e) {
   public Iterable<_DirectedEdge> adj(int v) {
     return adj[v];
   }
+
+  public Iterable<_DirectedEdge> edges() {
+    Bag<_DirectedEdge> bag = new Bag<>();
+    for (int v = 0; v < V; v++) {
+      for (_DirectedEdge e : adj[v]) {
+        bag.add(e);
+      }
+    }
+    return bag;
+  }
 }

Ch_4_4/note.md

@@ -0,0 +1,21 @@
+
+data structures for shortest path
+
+- parent-edge `edgeTo[v]`
+- distance to source `distTo[v]`, represent the length of sp from `s` to `v`
+
+![2020-05-12_000.jpg](https://gitee.com/gdhu/testtingop/raw/master/2020-05-12_000.jpg)
+
+edge relaxation
+
+![2020-05-12_001.jpg](https://gitee.com/gdhu/testtingop/raw/master/2020-05-12_001.jpg)
+
+if the length to w is larger than the sum of length to `v` plus weight of `v` to `w`,
+then `relax`: change `distTo[w] = distTo[v] + e.weight()` and `edgeTo[w] = e`.
+
+
+
+
+
+
+