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DijkstraCustomHeap.java
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DijkstraCustomHeap.java
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package Shortest_Paths;
import java.util.*;
import java.util.stream.Stream;
public class DijkstraCustomHeap {
public static void shortestPaths(List<Edge>[] edges, int s, long[] prio, int[] pred) {
Arrays.fill(pred, -1);
Arrays.fill(prio, Long.MAX_VALUE);
prio[s] = 0;
BinaryHeap h = new BinaryHeap(edges.length);
h.add(s, 0);
while (h.size != 0) {
int u = h.remove();
for (Edge e : edges[u]) {
int v = e.t;
long nprio = prio[u] + e.cost;
if (prio[v] > nprio) {
if (prio[v] == Long.MAX_VALUE)
h.add(v, nprio);
else
h.increasePriority(v, nprio);
prio[v] = nprio;
pred[v] = u;
}
}
}
}
public static class Edge {
int t;
int cost;
public Edge(int t, int cost) {
this.t = t;
this.cost = cost;
}
}
static class BinaryHeap {
long[] heap;
int[] pos2Id;
int[] id2Pos;
int size;
public BinaryHeap(int n) {
heap = new long[n];
pos2Id = new int[n];
id2Pos = new int[n];
}
public int remove() {
int removedId = pos2Id[0];
heap[0] = heap[--size];
pos2Id[0] = pos2Id[size];
id2Pos[pos2Id[0]] = 0;
down(0);
return removedId;
}
public void add(int id, long value) {
heap[size] = value;
pos2Id[size] = id;
id2Pos[id] = size;
up(size++);
}
public void increasePriority(int id, long value) {
heap[id2Pos[id]] = value;
up(id2Pos[id]);
}
void up(int pos) {
while (pos > 0) {
int parent = (pos - 1) / 2;
if (heap[pos] >= heap[parent])
break;
swap(pos, parent);
pos = parent;
}
}
void down(int pos) {
while (true) {
int child = 2 * pos + 1;
if (child >= size)
break;
if (child + 1 < size && heap[child + 1] < heap[child])
++child;
if (heap[pos] <= heap[child])
break;
swap(pos, child);
pos = child;
}
}
void swap(int i, int j) {
long tt = heap[i];
heap[i] = heap[j];
heap[j] = tt;
int t = pos2Id[i];
pos2Id[i] = pos2Id[j];
pos2Id[j] = t;
id2Pos[pos2Id[i]] = i;
id2Pos[pos2Id[j]] = j;
}
}
// Usage example
public static void main(String[] args) {
int[][] cost = {{0, 3, 2}, {0, 0, -2}, {0, 0, 0}};
int n = cost.length;
List<Edge>[] edges = Stream.generate(ArrayList::new).limit(n).toArray(List[]::new);
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
if (cost[i][j] != 0) {
edges[i].add(new Edge(j, cost[i][j]));
}
}
}
long[] dist = new long[n];
int[] pred = new int[n];
shortestPaths(edges, 0, dist, pred);
System.out.println(0 == dist[0]);
System.out.println(3 == dist[1]);
System.out.println(1 == dist[2]);
System.out.println(-1 == pred[0]);
System.out.println(0 == pred[1]);
System.out.println(1 == pred[2]);
}
}