Given a binary tree, print its nodes level by level. i.e. all nodes present at level 1 should be printed first followed by nodes of level 2 and so on.. All nodes for any level should be printed from left to right.

We have already discussed preorder, inorder and post-order traversals of the binary tree which are nothing but variations of Depth-first search of a Tree. Trees can also be traversed in level-order, where we visit every node on a level before going to a lower level. This search is referred to as level order traversal or breadth-first search (BFS), as the search tree is broadened as much as possible on each depth before going to the next depth.

Simple solution would be to print all nodes of level 1 first, followed by level 2, .. till level h where h is the height of the tree. All nodes present in a level can be printed by modifying pre-order traversal of the tree.

**C++ implementation –**

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// Data structure to store a Binary Tree node struct Node { int key; Node *left, *right; }; // Function to print all nodes of a given level from left to right bool printLevel(Node* root, int level) { if (root == nullptr) return false; if (level == 1) { cout << root->key << " "; // return true if at-least one node is present at given level return true; } bool left = printLevel(root->left, level - 1); bool right = printLevel(root->right, level - 1); return left || right; } // Function to print level order traversal of given binary tree void levelOrderTraversal(Node* root) { if (root == nullptr) return; // start from level 1 -- till height of the tree int level = 1; // run till printLevel() returns false while (printLevel(root, level)) level++; } |

The time complexity of above solution is O(n^{2}).

We can reduce time complexity to O(n) by using extra space. Below is pseudocode for a simple queue based level order traversal which require space proportional to the maximum number of nodes at a given depth. This can be as much as the total number of nodes / 2.

**levelorder(root)**

q -> empty queue

q.enqueue(root)

while (not q.isEmpty())

node -> q.dequeue()

visit(node)

if (node.left <> null)

q.enqueue(node.left)

if (node.right <> null)

q.enqueue(node.right)

**C++ implementation –**

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// Data structure to store a Binary Tree node struct Node { int key; Node *left, *right; }; // Function to print level order traversal of given binary tree void levelOrderTraversal(Node* root) { if (root == nullptr) return; // create an empty queue and enqueue root node list<Node*> queue; queue.push_back(root); // pointer to store current node Node* curr = nullptr; // run till queue is not empty while (queue.size()) { // process each node in queue and enqueue their // non-empty left and right child to queue curr = queue.front(); queue.pop_front(); cout << curr->key << " "; if (curr->left) queue.push_back(curr->left); if (curr->right) queue.push_back(curr->right); } } |

The time complexity of above solution is O(n) and auxiliary space used by the program is O(n).

We can also solve this problem by using Hashing. The idea is to traverse the tree in preorder fashion and store every node and its level into the multimap using level number as a key. Finally, we print all nodes corresponding to every level starting from first level. We can also traverse the tree in inorder or postorder fashion.

**C++ implementation –**

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// Data structure to store a Binary Tree node struct Node { int key; Node *left, *right; }; // traverse the tree in pre-order fashion and store nodes into the map // corresponding to their level void preorder(Node *root, int level, auto &map) { // base case: empty tree if (root == nullptr) return; // insert current node and its level into the map map[level].push_back(root->key); // recurse for left and right subtree by increasing level by 1 preorder(root->left, level + 1, map); preorder(root->right, level + 1, map); } // Recursive function to print level order traversal of given binary tree void levelOrderTraversal(Node *root) { // create an empty map to store nodes between given levels map<int, vector<int>> map; // traverse the tree and insert its nodes into the map // corresponding to the their level preorder(root, 1, map); // iterate through the map and print all nodes present in very level for (auto it: map) { cout << "Level " << it.first << ": "; for (int i: it.second) cout << i << " "; cout << endl; } } |

The time complexity of above solution is O(n) and auxiliary space used by the program is O(n).

**Exercise:**

Modify the solution to print nodes of different levels in separate line.

**References:**

https://en.wikipedia.org/wiki/Tree_traversal

**Thanks for reading.**

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