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We discussed about the tree traversal techniques in this post-

Please see pre-order traversal to understand the basics of visiting nodes. We have our same sample tree

Now let us try to understand the In-order traversal. In in-order traversal, the root is traversed between the sub trees. In-order traversal is defined as follows.

- Traverse the left sub-tree
- Visit the node
- Traverse the right sub-tree

So when we apply the in-order traversal on our example tree this will be done as:-

We get the final in-order traversal as:- **4, 2, 5, 1, 6, 3, 7**

The recursive version of in-order traversal can be written as:-

void inOrderRecursive(struct binaryTreeNode * root) { if(root) { // In-order traversal of left sub-tree (Step 1) inOrderRecursive(root -> left); // Visit the root (Step 2) printf("%d ", root -> data); // In-order traversal of right sub-tree(Step 3) inOrderRecursive(root -> right); } }

### Non-Recursive In-order Traversal

Non-recursive version of In-order traversal us very much similar to Pre-order. The only change is instead of processing the node before going to left sub-tree, process it after popping (which indicates after completion of left sub-tree processing).

void inOrderNonRecursive(struct binaryTreeNode * root) { // Create a new empty stack struct stackNode * S = NULL; // We need to loop until the stack is empty while(1) { // If the root is not null while(root) { // Push the first element onto the stack S = push(S, root); // Go to left sub-tree and keep on adding it to the stack root = root -> left; // (Step 1) } // Check if the stack is empty if(isStackEmpty(S)) break; // else we need to pop out the element root = S -> data; // Remove the element from the stack S = S -> next; // After popping process the current node printf("%d ",root -> data); // Step 2 // After printing, left sub-tree and root have been completed // Go to the right sub-tree root = root -> right; // Step 3 } // We need to delete the stack free(S); }

*Time Complexity:-* O(n)

*Space Complexity:-* O(n)

You can download the complete working code here.