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Get First node at a given level in a BT

Can you please help with C code to get first node in a binary tree given a particular level .

asked Feb 19, 2016 by anonymous

1 Answer

I have made small changes for printing first node, in level order traversal code sourced from here :

and now it prints first node of specified level :

// Recursive C program for level order traversal of Binary Tree
#include <stdio.h>
#include <stdlib.h>
/* A binary tree node has data, pointer to left child
   and a pointer to right child */
struct node
    int data;
    struct node* left, *right;
/* Function protoypes */
void printGivenLevel(struct node* root, int level);
int height(struct node* node);
struct node* newNode(int data);
int firstNode;
/* Function to print level order traversal a tree*/
void printLevelOrder(struct node* root)
    int h = height(root);
    int i;
    for (i=1; i<=h; i++)
        printGivenLevel(root, i);
/* Print nodes at a given level */
void printGivenLevel(struct node* root, int level)
    if (root == NULL)
    if (level == 1)
        //	Just store first node,    it will be -1 if not assigned yet
    	if (firstNode == -1)						
	    	firstNode = root->data;
    else if (level > 1)
        printGivenLevel(root->left, level-1);
        printGivenLevel(root->right, level-1);
/* Compute the "height" of a tree -- the number of
    nodes along the longest path from the root node
    down to the farthest leaf node.*/
int height(struct node* node)
    if (node==NULL)
        return 0;
        /* compute the height of each subtree */
        int lheight = height(node->left);
        int rheight = height(node->right);
        /* use the larger one */
        if (lheight > rheight)
        else return(rheight+1);
/* Helper function that allocates a new node with the
   given data and NULL left and right pointers. */
struct node* newNode(int data)
    struct node* node = (struct node*)malloc(sizeof(struct node));
    node->data = data;
    node->left = NULL;
    node->right = NULL;
/* Driver program to test above functions*/
int main()
    struct node *root = newNode(1);
    root->left        = newNode(2);
    root->right       = newNode(3);
    root->left->left  = newNode(4);
    root->left->right = newNode(5);
    printf("Level Order traversal of binary tree is \n");
    firstNode = -1;
    printGivenLevel(root, 2);
    printf("%d\n", firstNode);
    return 0;


answered Feb 19, 2016 by utkarsh111