Create zero_one_bfs.java (#6560)

* Create zero_one_bfs.java

* Rename zero_one_bfs.java to ZeroOneBfs.java

* Update ZeroOneBfs.java

* Update ZeroOneBfs.java

* Create ZeroOneBfsTest.java

* Update ZeroOneBfsTest.java

---------

Co-authored-by: a <[email protected]>

Author: ananya-research

src/main/java/com/thealgorithms/graph/ZeroOneBfs.java

@@ -0,0 +1,77 @@
+package com.thealgorithms.graph;
+
+import java.util.ArrayDeque;
+import java.util.Arrays;
+import java.util.Deque;
+import java.util.List;
+
+/**
+ * 0-1 BFS for shortest paths on graphs with edges weighted 0 or 1.
+ *
+ * <p>Time Complexity: O(V + E). Space Complexity: O(V).
+ *
+ * <p>References:
+ * <ul>
+ *   <li>https://cp-algorithms.com/graph/01_bfs.html</li>
+ * </ul>
+ */
+public final class ZeroOneBfs {
+
+    private ZeroOneBfs() {
+        // Utility class; do not instantiate.
+    }
+
+    /**
+     * Computes shortest distances from {@code src} in a graph whose edges have weight 0 or 1.
+     *
+     * @param n the number of vertices, labeled {@code 0..n-1}
+     * @param adj adjacency list; for each vertex u, {@code adj.get(u)} is a list of pairs
+     *            {@code (v, w)} where {@code v} is a neighbor and {@code w} is 0 or 1
+     * @param src the source vertex
+     * @return an array of distances; {@code Integer.MAX_VALUE} denotes unreachable
+     * @throws IllegalArgumentException if {@code n < 0}, {@code src} is out of range,
+     *                                  or any edge has weight other than 0 or 1
+     */
+    public static int[] shortestPaths(int n, List<List<int[]>> adj, int src) {
+        if (n < 0 || src < 0 || src >= n) {
+            throw new IllegalArgumentException("Invalid n or src");
+        }
+        int[] dist = new int[n];
+        Arrays.fill(dist, Integer.MAX_VALUE);
+        Deque<Integer> dq = new ArrayDeque<>();
+
+        dist[src] = 0;
+        dq.addFirst(src);
+
+        while (!dq.isEmpty()) {
+            int u = dq.pollFirst();
+            List<int[]> edges = adj.get(u);
+            if (edges == null) {
+                continue;
+            }
+            for (int[] e : edges) {
+                if (e == null || e.length < 2) {
+                    continue;
+                }
+                int v = e[0];
+                int w = e[1];
+                if (v < 0 || v >= n) {
+                    continue; // ignore bad edges
+                }
+                if (w != 0 && w != 1) {
+                    throw new IllegalArgumentException("Edge weight must be 0 or 1");
+                }
+                int nd = dist[u] + w;
+                if (nd < dist[v]) {
+                    dist[v] = nd;
+                    if (w == 0) {
+                        dq.addFirst(v);
+                    } else {
+                        dq.addLast(v);
+                    }
+                }
+            }
+        }
+        return dist;
+    }
+}

src/test/java/com/thealgorithms/graph/ZeroOneBfsTest.java

@@ -0,0 +1,68 @@
+package com.thealgorithms.graph;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.ArrayList;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class ZeroOneBfsTest {
+
+    // Helper to build adjacency list with capacity n
+    private static List<List<int[]>> makeAdj(int n) {
+        List<List<int[]>> adj = new ArrayList<>(n);
+        for (int i = 0; i < n; i++) {
+            adj.add(new ArrayList<>());
+        }
+        return adj;
+    }
+
+    @Test
+    void simpleLineGraph() {
+        int n = 4;
+        List<List<int[]>> adj = makeAdj(n);
+        // 0 --0--> 1 --1--> 2 --0--> 3
+        adj.get(0).add(new int[] {1, 0});
+        adj.get(1).add(new int[] {2, 1});
+        adj.get(2).add(new int[] {3, 0});
+
+        int[] dist = ZeroOneBfs.shortestPaths(n, adj, 0);
+        assertArrayEquals(new int[] {0, 0, 1, 1}, dist);
+    }
+
+    @Test
+    void parallelEdgesPreferZero() {
+        int n = 3;
+        List<List<int[]>> adj = makeAdj(n);
+        // Two edges 0->1: weight 1 and weight 0. Algorithm should choose 0.
+        adj.get(0).add(new int[] {1, 1});
+        adj.get(0).add(new int[] {1, 0});
+        adj.get(1).add(new int[] {2, 1});
+
+        int[] dist = ZeroOneBfs.shortestPaths(n, adj, 0);
+        assertArrayEquals(new int[] {0, 0, 1}, dist);
+    }
+
+    @Test
+    void unreachableNodes() {
+        int n = 3;
+        List<List<int[]>> adj = makeAdj(n);
+        adj.get(0).add(new int[] {1, 0});
+        int[] dist = ZeroOneBfs.shortestPaths(n, adj, 0);
+        // node 2 unreachable -> Integer.MAX_VALUE
+        assertArrayEquals(new int[] {0, 0, Integer.MAX_VALUE}, dist);
+    }
+
+    @Test
+    void invalidArgs() {
+        int n = 2;
+        List<List<int[]>> adj = makeAdj(n);
+        // invalid weight
+        adj.get(0).add(new int[] {1, 2});
+        assertThrows(IllegalArgumentException.class, () -> ZeroOneBfs.shortestPaths(n, adj, 0));
+        // invalid src
+        assertThrows(IllegalArgumentException.class, () -> ZeroOneBfs.shortestPaths(n, adj, -1));
+        assertThrows(IllegalArgumentException.class, () -> ZeroOneBfs.shortestPaths(n, adj, 2));
+    }
+}