The following C program, using iteration, performs a Depth First Search traversal. Depth-first search (DFS) is an algorithm for traversing or searching a tree, tree structure or graph. The concept of backtracking is used in DFS. In this program we are performing DFS on a binary tree. In DFS, the deepest and univisited node is visited and backtracks to it’s root if no siblings of that node exists.
Conditions: The DFS works on acyclic graph. DFS may fail if it enters a cycle. Care must be taken by not extending a path to a node if it already has.

Here is the source code of the C program to apply DFS on a binary tree iteratively. The C program is successfully compiled and run on a Linux system. The program output is also shown below.



/*
* C Program for Depth First Binary Tree Search without using
* Recursion
*/
#include <stdio.h>
#include <stdlib.h>

struct node
{
int a;
struct node *left;
struct node *right;
int visited;
};

void generate(struct node **, int);
void DFS(struct node *);
void delete(struct node **);

int main()
{
struct node *head = NULL;
int choice = 0, num, flag = 0, key;

do
{
printf("\nEnter your choice:\n1. Insert\n2. Perform DFS Traversal\n3. Exit\nChoice: ");
scanf("%d", &choice);
switch(choice)
{
case 1:
printf("Enter element to insert: ");
scanf("%d", &num);
generate(&head, num);
break;
case 2:
DFS(head);
break;
case 3:
delete(&head);
printf("Memory Cleared\nPROGRAM TERMINATED\n");
break;
default:
printf("Not a valid input, try again\n");
}
} while (choice != 3);

return 0;
}

void generate(struct node **head, int num)
{
struct node *temp = *head, *prev = *head;

if (*head == NULL)
{
*head = (struct node *)malloc(sizeof(struct node));
(*head)->a = num;
(*head)->visited = 0;
(*head)->left = (*head)->right = NULL;
}
else
{
while (temp != NULL)
{
if (num > temp->a)
{
prev = temp;
temp = temp->right;
}
else
{
prev = temp;
temp = temp->left;
}
}
temp = (struct node *)malloc(sizeof(struct node));
temp->a = num;
temp->visited = 0;
if (temp->a >= prev->a)
{
prev->right = temp;
}
else
{
prev->left = temp;
}
}
}

void DFS(struct node *head)
{
struct node *temp = head, *prev;

printf("On DFS traversal we get:\n");
while (temp && !temp->visited)
{
if (temp->left && !temp->left->visited)
{
temp = temp->left;
}
else if (temp->right && !temp->right->visited)
{
temp = temp->right;
}
else
{
printf("%d ", temp->a);
temp->visited = 1;
temp = head;
}
}
}

void delete(struct node **head)
{
if (*head != NULL)
{
if ((*head)->left)
{
delete(&(*head)->left);
}
if ((*head)->right)
{
delete(&(*head)->right);
}
free(*head);
}
}
$ cc pgm33.c
$ a.out

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 1
Enter element to insert: 5

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 1
Enter element to insert: 3

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 1
Enter element to insert: 2

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 1
Enter element to insert: 4

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 1
Enter element to insert: 1

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 1
Enter element to insert: 7

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 1
Enter element to insert: 6

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 1
Enter element to insert: 8

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 2
On DFS traversal we get:
1 2 4 3 6 8 7 5

Enter your choice:
1. Insert
2. Perform DFS Traversal
3. Exit
Choice: 3
Memory Cleared
PROGRAM TERMINATED