Go golang clean solution
Runtime: 16 ms, faster than 61.39% of Go online submissions for Path Sum III.\nMemory Usage: 4.3 MB, less than 100.00% of Go online submissions for Path Sum III
>Runtime: 16 ms, faster than 61.39% of Go online submissions for Path Sum III.\nMemory Usage: 4.3 MB, less than 100.00% of Go online submissions for Path Sum III.\n\n```go\nfunc pathSum(root *TreeNode, sum int) int {\n    if root == nil { return 0 }\n    return helper(root, sum) + pathSum(root.Left, sum) + pathSum(root.Right, sum)\n}\n\nfunc helper(root *TreeNode, sum int) int {\n    count := 0\n    if root == nil { return 0 }\n    if root.Val == sum { count++ }\n    return count + helper(root.Left, sum - root.Val) + helper(root.Right, sum - root.Val)\n}\n```
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