Added solution to find build order given projects and dependencies

YogenRaii · YogenRaii · commit 8e94907fec9e · 2019-06-05T21:43:41.000-05:00
diff --git a/src/main/java/com/eprogrammerz/examples/algorithm/graphs/BuildOrder.java b/src/main/java/com/eprogrammerz/examples/algorithm/graphs/BuildOrder.java
@@ -0,0 +1,85 @@
+package com.eprogrammerz.examples.algorithm.graphs;
+
+import org.junit.Test;
+
+import java.util.*;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * You are given a list of projects and a list of dependencies (which is a list of pairs of projects,
+ * where the second project is dependent on the first project).
+ * All of a project's dependencies must be built before the project is.
+ * Find a build order that will allow the projects to be built. If there is no valid build order, return an error.
+ * EXAMPLE
+ * Input:
+ * projects: a, b, c, d, e, f
+ * dependencies: (a, d), (f, b), (b, d), (f, a), (d, c) Output: f, e, a, b, d, c
+ */
+public class BuildOrder {
+    List<Integer> findBuildOrder(List<Integer> projects, List<Pair> dependencies) {
+        Map<Integer, Set<Integer>> projectToDependencies = new HashMap<>();
+
+        // initialize project to empty set
+        for (int project : projects) {
+            projectToDependencies.put(project, new HashSet<>());
+        }
+
+        // set project to dependencies
+        for (Pair pair : dependencies) {
+            Set<Integer> dep = projectToDependencies.get(pair.y);
+            dep.add(pair.x);
+        }
+
+        // adj list representation of dependencies
+//        System.out.println(projectToDependencies); // {1=[6], 2=[6], 3=[4], 4=[1, 2], 5=[], 6=[]}
+        // build order storing variable
+        List<Integer> buildOrder = new ArrayList<>();
+
+        // to track the completed ones
+        Queue<Integer> toBeBuilt = new LinkedList<>();
+        projectToDependencies.forEach((p, d) -> {
+            if (d.isEmpty()) {
+                toBeBuilt.add(p);
+            }
+        });
+
+        while (!toBeBuilt.isEmpty()) {
+            int ready = toBeBuilt.poll();
+            buildOrder.add(ready);
+
+            // scan this to each dependencies
+            // if found, remove that
+            // after removal, if empty
+            // add that to queue
+
+            projectToDependencies.forEach((p, d) -> {
+                if (d.remove(ready) && d.isEmpty()) {
+                    toBeBuilt.add(p);
+                }
+            });
+        }
+
+        return buildOrder;
+    }
+
+    class Pair {
+        int x;
+        int y;
+
+        Pair(int x, int y) {
+            this.x = x;
+            this.y = y;
+        }
+    }
+
+    @Test
+    public void testFindBuildOrder() {
+
+        List<Integer> projects = Arrays.asList(1, 2, 3, 4, 5, 6);
+        List<Pair> deps = Arrays.asList(new Pair(1, 4), new Pair(2, 4), new Pair(6, 2), new Pair(6, 1), new Pair(4, 3));
+        List<Integer> buildOrder = findBuildOrder(projects, deps);
+        List<Integer> expected = Arrays.asList(5, 6, 1, 2, 4, 3);
+        assertEquals(expected, buildOrder);
+    }
+}
