Sum of Leaf Nodes in a Binary Tree

You are given a binary tree. A leaf node is a node with no children. Your task is to compute the sum of all leaf nodes in the tree.

Let \(T\) be a binary tree and \(L\) be the set of its leaves. The sum of leaf nodes is given by:

\[ S = \sum_{v \in L} v \]

If the tree is empty, then \(S = 0\).

inputFormat

The input consists of a single line containing comma-separated tokens. Each token is either an integer or the string null, representing the nodes of the binary tree in level order. The first token is the value of the root. For a missing child, the token null is used.

Examples:

• 1,2,3,4,5,6,7 represents a tree where the leaves are 4, 5, 6, and 7.
• 10,8,2,null,null,3,6 represents a tree with leaves 8, 3, and 6.
• null represents an empty tree.

outputFormat

Output a single integer, which is the sum of all leaf nodes in the binary tree.

## sample

1,2,3,4,5,6,7

22

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