feat: Backtracking M-Coloring Problem Algorithm

backtracking/m_coloring_problem.py

@@ -0,0 +1,78 @@
+def is_safe(
+    node: int,
+    color: int,
+    graph: list[list[int]],
+    num_vertices: int,
+    col: list[int],
+) -> bool:
+    """
+    Check if it is safe to assign a color to a node.
+
+    >>> is_safe(0, 1, [[0,1],[1,0]], 2, [0,1])
+    False
+    >>> is_safe(0, 2, [[0,1],[1,0]], 2, [0,1])
+    True
+    """
+    return all(
+        not (graph[node][k] == 1 and col[k] == color) for k in range(num_vertices)
+    )
+
+
+def solve(
+    node: int,
+    col: list[int],
+    max_colors: int,
+    num_vertices: int,
+    graph: list[list[int]],
+) -> bool:
+    """
+    Recursively try to color the graph using at most max_colors.
+
+    >>> solve(0, [0]*3, 3, 3, [[0,1,0],[1,0,1],[0,1,0]])
+    True
+    >>> solve(0, [0]*3, 2, 3, [[0,1,0],[1,0,1],[0,1,0]])
+    False
+    """
+    if node == num_vertices:
+        return True
+    for c in range(1, max_colors + 1):
+        if is_safe(node, c, graph, num_vertices, col):
+            col[node] = c
+            if solve(node + 1, col, max_colors, num_vertices, graph):
+                return True
+            col[node] = 0
+    return False
+
+
+def graph_coloring(graph: list[list[int]], max_colors: int, num_vertices: int) -> bool:
+    """
+    Determine if the graph can be colored with at most max_colors.
+
+    >>> graph_coloring([[0,1,1],[1,0,1],[1,1,0]], 3, 3)
+    True
+    >>> graph_coloring([[0,1,1],[1,0,1],[1,1,0]], 2, 3)
+    False
+    """
+    col = [0] * num_vertices
+    return solve(0, col, max_colors, num_vertices, graph)
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
+    num_vertices = int(input("Enter number of vertices: "))
+    num_edges = int(input("Enter number of edges: "))
+    graph = [[0] * num_vertices for _ in range(num_vertices)]
+
+    for _ in range(num_edges):
+        u, v = map(int, input().split())
+        if 0 <= u < num_vertices and 0 <= v < num_vertices:
+            graph[u][v] = 1
+            graph[v][u] = 1
+        else:
+            raise ValueError("Edge indices out of range")
+
+    max_colors = int(input("Enter maximum number of colors: "))
+    print(graph_coloring(graph, max_colors, num_vertices))