# Sum of leaf nodes at minimum level

Given a binary tree containing n nodes. The problem is to get the sum of all the leaf nodes which are at minimum level in the binary tree.

Examples:

```Input :
1
/   \
2     3
/  \   /  \
4   5   6   7
/     \
8       9

Output : 11
Leaf nodes 4 and 7 are at minimum level.
Their sum = (4 + 7) = 11.
```

## Recommended: Please solve it on PRACTICE first, before moving on to the solution.

Approach: Perform iterative level order traversal using queue and find the first level containing a leaf node. Sum up all the leaf nodes at this level and then stop performing the traversal further.

## C++

 `// C++ implementation to find the sum of ` `// leaf nodes at minimum level ` `#include ` `using` `namespace` `std; ` ` `  `// structure of a node of binary tree ` `struct` `Node { ` `    ``int` `data; ` `    ``Node *left, *right; ` `}; ` ` `  `// function to get a new node ` `Node* getNode(``int` `data) ` `{ ` `    ``// allocate space ` `    ``Node* newNode = (Node*)``malloc``(``sizeof``(Node)); ` ` `  `    ``// put in the data ` `    ``newNode->data = data; ` `    ``newNode->left = newNode->right = NULL; ` `    ``return` `newNode; ` `} ` ` `  `// function to find the sum of ` `// leaf nodes at minimum level ` `int` `sumOfLeafNodesAtMinLevel(Node* root) ` `{ ` `    ``// if tree is empty ` `    ``if` `(!root) ` `        ``return` `0; ` ` `  `    ``// if there is only one node ` `    ``if` `(!root->left && !root->right) ` `        ``return` `root->data; ` ` `  `    ``// queue used for level order traversal ` `    ``queue q; ` `    ``int` `sum = 0;  ` `    ``bool` `f = 0; ` ` `  `    ``// push root node in the queue 'q' ` `    ``q.push(root); ` ` `  `    ``while` `(f == 0) { ` ` `  `        ``// count number of nodes in the ` `        ``// current level ` `        ``int` `nc = q.size(); ` ` `  `        ``// traverse the current level nodes ` `        ``while` `(nc--) { ` ` `  `            ``// get front element from 'q' ` `            ``Node* top = q.front(); ` `            ``q.pop(); ` ` `  `            ``// if it is a leaf node ` `            ``if` `(!top->left && !top->right) { ` ` `  `                ``// accumulate data to 'sum' ` `                ``sum += top->data; ` ` `  `                ``// set flag 'f' to 1, to signify  ` `                ``// minimum level for leaf nodes  ` `                ``// has been encountered ` `                ``f = 1; ` `            ``} ` `            ``else` `{ ` ` `  `                ``// if top's left and right child  ` `                ``// exists, then push them to 'q' ` `                ``if` `(top->left) ` `                    ``q.push(top->left); ` `                ``if` `(top->right) ` `                    ``q.push(top->right); ` `            ``} ` `        ``} ` `    ``} ` ` `  `    ``// required sum ` `    ``return` `sum; ` `} ` ` `  `// Driver program to test above ` `int` `main() ` `{ ` `    ``// binary tree creation ` `    ``Node* root = getNode(1); ` `    ``root->left = getNode(2); ` `    ``root->right = getNode(3); ` `    ``root->left->left = getNode(4); ` `    ``root->left->right = getNode(5); ` `    ``root->right->left = getNode(6); ` `    ``root->right->right = getNode(7); ` `    ``root->left->right->left = getNode(8); ` `    ``root->right->left->right = getNode(9); ` ` `  `    ``cout << ``"Sum = "` `         ``<< sumOfLeafNodesAtMinLevel(root); ` ` `  `    ``return` `0; ` `} `

## Java

 `// Java implementation to find the sum of ` `// leaf nodes at minimum level ` `import` `java.util.*; ` ` `  `class` `GFG  ` `{ ` ` `  `// structure of a node of binary tree ` `static` `class` `Node  ` `{ ` `    ``int` `data; ` `    ``Node left, right; ` `}; ` ` `  `// function to get a new node ` `static` `Node getNode(``int` `data) ` `{ ` `    ``// allocate space ` `    ``Node newNode = ``new` `Node(); ` ` `  `    ``// put in the data ` `    ``newNode.data = data; ` `    ``newNode.left = newNode.right = ``null``; ` `    ``return` `newNode; ` `} ` ` `  `// function to find the sum of ` `// leaf nodes at minimum level ` `static` `int` `sumOfLeafNodesAtMinLevel(Node root) ` `{ ` `    ``// if tree is empty ` `    ``if` `(root == ``null``) ` `        ``return` `0``; ` ` `  `    ``// if there is only one node ` `    ``if` `(root.left == ``null` `&& ` `        ``root.right == ``null``) ` `        ``return` `root.data; ` ` `  `    ``// queue used for level order traversal ` `    ``Queue q = ``new` `LinkedList<>(); ` `    ``int` `sum = ``0``;  ` `    ``boolean` `f = ``false``; ` ` `  `    ``// push root node in the queue 'q' ` `    ``q.add(root); ` ` `  `    ``while` `(f == ``false``) ` `    ``{ ` ` `  `        ``// count number of nodes in the ` `        ``// current level ` `        ``int` `nc = q.size(); ` ` `  `        ``// traverse the current level nodes ` `        ``while` `(nc-- >``0``) ` `        ``{ ` ` `  `            ``// get front element from 'q' ` `            ``Node top = q.peek(); ` `            ``q.remove(); ` ` `  `            ``// if it is a leaf node ` `            ``if` `(top.left == ``null` `&&  ` `                ``top.right == ``null``)  ` `            ``{ ` ` `  `                ``// accumulate data to 'sum' ` `                ``sum += top.data; ` ` `  `                ``// set flag 'f' to 1, to signify  ` `                ``// minimum level for leaf nodes  ` `                ``// has been encountered ` `                ``f = ``true``; ` `            ``} ` `            ``else`  `            ``{ ` ` `  `                ``// if top's left and right child  ` `                ``// exists, then push them to 'q' ` `                ``if` `(top.left != ``null``) ` `                    ``q.add(top.left); ` `                ``if` `(top.right != ``null``) ` `                    ``q.add(top.right); ` `            ``} ` `        ``} ` `    ``} ` ` `  `    ``// required sum ` `    ``return` `sum; ` `} ` ` `  `// Driver Code ` `public` `static` `void` `main(String[] args) ` `{ ` `     `  `    ``// binary tree creation ` `    ``Node root = getNode(``1``); ` `    ``root.left = getNode(``2``); ` `    ``root.right = getNode(``3``); ` `    ``root.left.left = getNode(``4``); ` `    ``root.left.right = getNode(``5``); ` `    ``root.right.left = getNode(``6``); ` `    ``root.right.right = getNode(``7``); ` `    ``root.left.right.left = getNode(``8``); ` `    ``root.right.left.right = getNode(``9``); ` ` `  `    ``System.out.println(``"Sum = "` `+  ` `           ``sumOfLeafNodesAtMinLevel(root)); ` `    ``} ` `} ` ` `  `// This code is contributed by Rajput-Ji `

## C#

 `// C# implementation to find the sum of ` `// leaf nodes at minimum level ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `class` `GFG  ` `{ ` ` `  `// structure of a node of binary tree ` `class` `Node  ` `{ ` `    ``public` `int` `data; ` `    ``public` `Node left, right; ` `}; ` ` `  `// function to get a new node ` `static` `Node getNode(``int` `data) ` `{ ` `    ``// allocate space ` `    ``Node newNode = ``new` `Node(); ` ` `  `    ``// put in the data ` `    ``newNode.data = data; ` `    ``newNode.left = newNode.right = ``null``; ` `    ``return` `newNode; ` `} ` ` `  `// function to find the sum of ` `// leaf nodes at minimum level ` `static` `int` `sumOfLeafNodesAtMinLevel(Node root) ` `{ ` `    ``// if tree is empty ` `    ``if` `(root == ``null``) ` `        ``return` `0; ` ` `  `    ``// if there is only one node ` `    ``if` `(root.left == ``null` `&& ` `        ``root.right == ``null``) ` `        ``return` `root.data; ` ` `  `    ``// queue used for level order traversal ` `    ``Queue q = ``new` `Queue(); ` `    ``int` `sum = 0;  ` `    ``bool` `f = ``false``; ` ` `  `    ``// push root node in the queue 'q' ` `    ``q.Enqueue(root); ` ` `  `    ``while` `(f == ``false``) ` `    ``{ ` ` `  `        ``// count number of nodes in the ` `        ``// current level ` `        ``int` `nc = q.Count; ` ` `  `        ``// traverse the current level nodes ` `        ``while` `(nc-- >0) ` `        ``{ ` ` `  `            ``// get front element from 'q' ` `            ``Node top = q.Peek(); ` `            ``q.Dequeue(); ` ` `  `            ``// if it is a leaf node ` `            ``if` `(top.left == ``null` `&&  ` `                ``top.right == ``null``)  ` `            ``{ ` ` `  `                ``// accumulate data to 'sum' ` `                ``sum += top.data; ` ` `  `                ``// set flag 'f' to 1, to signify  ` `                ``// minimum level for leaf nodes  ` `                ``// has been encountered ` `                ``f = ``true``; ` `            ``} ` `            ``else` `            ``{ ` ` `  `                ``// if top's left and right child  ` `                ``// exists, then push them to 'q' ` `                ``if` `(top.left != ``null``) ` `                    ``q.Enqueue(top.left); ` `                ``if` `(top.right != ``null``) ` `                    ``q.Enqueue(top.right); ` `            ``} ` `        ``} ` `    ``} ` ` `  `    ``// required sum ` `    ``return` `sum; ` `} ` ` `  `// Driver Code ` `public` `static` `void` `Main(String[] args) ` `{ ` `     `  `    ``// binary tree creation ` `    ``Node root = getNode(1); ` `    ``root.left = getNode(2); ` `    ``root.right = getNode(3); ` `    ``root.left.left = getNode(4); ` `    ``root.left.right = getNode(5); ` `    ``root.right.left = getNode(6); ` `    ``root.right.right = getNode(7); ` `    ``root.left.right.left = getNode(8); ` `    ``root.right.left.right = getNode(9); ` ` `  `    ``Console.WriteLine(``"Sum = "` `+  ` `            ``sumOfLeafNodesAtMinLevel(root)); ` `    ``} ` `} ` ` `  `// This code is contributed by PrinciRaj1992 `

Output:

```Sum = 11
```

Time Complexity: O(n).
Auxiliary Space: O(n).

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