Add topological sort (#117)

sorting_algorithms/topological_sort.r

@@ -0,0 +1,49 @@
+# Function to perform topological sort
+topological_sort <- function(graph) {
+  # Number of vertices in the graph
+  num_vertices <- length(graph)
+
+  # Helper function to perform DFS
+  dfs <- function(node, visited, stack) {
+    visited[node] <- TRUE
+
+    # Visit all adjacent vertices
+    for (neighbor in graph[[node]]) {
+      if (!visited[neighbor]) {
+        dfs(neighbor, visited, stack)
+      }
+    }
+
+    # Push the current node onto the stack
+    stack <<- c(stack, node)
+  }
+
+  # Initialize data structures
+  visited <- rep(FALSE, num_vertices)
+  stack <- c()
+
+  # Perform DFS for each unvisited vertex
+  for (node in 1:num_vertices) {
+    if (!visited[node]) {
+      dfs(node, visited, stack)
+    }
+  }
+
+  # Reverse the stack to get the topological order
+  topological_order <- rev(stack)
+  return(topological_order)
+}
+
+# Example usage
+# Define a sample DAG as an adjacency list
+# Here, we represent the graph as a list of vectors, where each vector contains the neighbors of the corresponding node.
+graph <- list(
+  c(2, 3),    # Node 1 points to nodes 2 and 3
+  c(3, 4),    # Node 2 points to nodes 3 and 4
+  c(5),       # Node 3 points to node 5
+  c(5),       # Node 4 points to node 5
+  numeric(0)  # Node 5 has no outgoing edges
+)
+
+topological_order <- topological_sort(graph)
+cat("Topological Order:", topological_order, "\n")