DATA STRUCTURES PROGRAM • GRAPHS
Topologically Sort a DAG with DFS
Learn how to topologically sort a dag with dfs using a clear C program.
PROBLEM UNDERSTANDING
Input and expected output
Sample input
No input required
Sample output
5 4 2 3 1 0
COMPLETE C PROGRAM
Complete C implementation
#include <stdio.h>
void topo(int vertex, int graph[6][6], int visited[6], int stack[6], int *top)
{
visited[vertex] = 1;
for (int neighbour = 0; neighbour < 6; neighbour++)
if (graph[vertex][neighbour] && !visited[neighbour]) topo(neighbour, graph, visited, stack, top);
stack[(*top)++] = vertex;
}
int main(void)
{
int graph[6][6] = {{0}};
int edges[][2] = {{5,2},{5,0},{4,0},{4,1},{2,3},{3,1}};
for (int index = 0; index < 6; index++) graph[edges[index][0]][edges[index][1]] = 1;
int visited[6] = {0}, stack[6], top = 0;
for (int vertex = 0; vertex < 6; vertex++)
if (!visited[vertex]) topo(vertex, graph, visited, stack, &top);
while (top > 0) printf("%d ", stack[--top]);
putchar('\n');
return 0;
}CURRENT STEP
Select Start to walk through the important lines.
SELECTED LINE
No line selected
EXPECTED OUTPUT FOR THE SAMPLE
5 4 2 3 1 0
Step 0 of 0
PROGRAM EXPLANATION
Algorithm and explanation
- Read the required input values.
- Push each vertex after all descendants finish, then reverse the finish order.
- Display the computed result.
Push each vertex after all descendants finish, then reverse the finish order.
EFFICIENCY
Time and space complexity
Time complexity
O(V²) with matrix
Auxiliary space
O(V)
DEBUGGING CHECKLIST
Common mistakes
Check this
Use the correct format specifier for every variable.
Check this
Initialize variables before using their values.
Check this
Check braces, semicolons and input order carefully.
Try it yourself
Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.