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BFS of binary tree

Question : Write a function for BFS traversal of a binary tree.


Bread first traversal (also called level order traversal) is a traversal method where you visit the siblings of a node before you visit its descendants.

The other method of traversal is called depth first traversal(DFS) where you visit the descendants of a node before you visit its siblings. In-order, pre-order and post-order are all DFS traversal methods.

In BFS, first you visit all the nodes at level 0, then you visit all the nodes at level 1, then you visit nodes at level 2 etc.
For the diagram shown above, the BFS output should be
8
3 10
1 6 14 
4 7 13  ( there is no newline between levels)

To write a code for BFS, we need to take the help of another data structure - a queue.

  • To start with we insert this root to the queue.
  • Then as long as queue is not empty, we remove a node from the queue,
  • visit this node (display its value)
  • and enqueue its both child nodes. 
Here is C function for BFS


void bfs_traverse(NODEPTR root)
{
    struct queue q1;
    NODEPTR nd;
    q1.front=0;q1.rear =-1;
    enqueue(&q1,root);
    while(!is_empty(q1))
    {
        nd = dequeue(&q1) ;
        printf("%d  ",nd->value);     
        if (nd->left != NULL)
            enqueue(&q1,nd->left);
        if (nd->right != NULL )
            enqueue(&q1,nd->right);         
    }  
}

Here is complete program to display the nodes of a binary search tree in Bread first traversal. You can download this program from here. 


#include<stdio.h>
#include<stdlib.h>
struct btnode
{ 
    int value; 
    struct btnode *left, *right; 
};
typedef struct btnode *NODEPTR ;
#define MAX 40
struct queue
{ 
   NODEPTR nodes[40];
   int rear,front;
}; 

 NODEPTR insert_node(NODEPTR nd,NODEPTR newnode)
{
    if(nd==NULL)
       return newnode;/* newnode becomes root of tree*/
    if(newnode->value > nd->value)
        nd->right = insert_node(nd->right,newnode);
    else if(newnode->value <  nd->value)
        nd->left = insert_node(nd->left,newnode); 
    return nd;   
}
 
NODEPTR create_node(int num)
{
     NODEPTR temp = (NODEPTR)malloc(sizeof(struct btnode));
     temp->value = num;
     temp->left = NULL;
     temp->right = NULL;
     return temp;
} 
void enqueue(struct queue *qptr,NODEPTR newnode)
{
    if(qptr->rear>=MAX)
      {
 printf("Queue overflow");
        return;
      }
    qptr->rear++;
    qptr->nodes[qptr->rear]=newnode;
}
int is_empty(struct queue qptr)
{
    if (qptr.front>qptr.rear)
      return 1;
    return 0;
}
  
NODEPTR dequeue(struct queue *qptr)
{
    if(is_empty(*qptr))
    {
 printf("Queue empty");
        return NULL;
    }    
    NODEPTR temp= qptr->nodes[qptr->front];
    qptr->front++;
    return temp;
}
   
/* displaying elements using BFS traversal */

void bfs_traverse(NODEPTR root)
{
    struct queue q1;
    NODEPTR nd;
    q1.front=0;q1.rear =-1;
    enqueue(&q1,root);
    while(!is_empty(q1))
    {
        nd = dequeue(&q1) ;
        printf("%d  ",nd->value);     
        if (nd->left != NULL)
            enqueue(&q1,nd->left);
        if (nd->right != NULL )
            enqueue(&q1,nd->right);         
    }  
}

int main() 
{ 
    NODEPTR root = NULL,newnode ; 
    int num = 1; 
    printf("Enter the elements of the tree(enter -1 to exit)\n"); 

    while (1) 
    {     
        scanf("%d",  &num); 
        if (num  ==  -1) 
            break; 
        newnode = create_node(num);
        root = insert_node(root,newnode);
    }
    printf("elements in bfs are\n"); 
    bfs_traverse(root);
    
}
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