Sum of Left Leaves in a Binary Tree

You are given a binary tree represented in level‐order as a comma-separated string, where the value null represents a missing node. Your task is to compute the sum of all left leaves in the tree. A left leaf is a node that:

• Is a left child of its parent
• Does not have any children (i.e. it is a leaf node)

The input is provided in a single line and the output should be a single integer. Use any standard tree traversal technique (such as depth-first search) to solve the problem.

Note: In a binary tree, a node’s left child is considered a left leaf only if it is a leaf node. For example, consider the binary tree below:

3
├── 9
└── 20
    ├── 15
    └── 7

Here, the left leaves are 9 (left child of 3) and 15 (left child of 20), so the sum is 9 + 15 = 24.

inputFormat

The input consists of a single line containing comma-separated values that represent the nodes of the binary tree in level-order. The string null represents a missing node.

Example: 3,9,20,null,null,15,7

outputFormat

Output a single integer which is the sum of all left leaves in the binary tree.

## sample

3,9,20,null,null,15,7

24