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Without using a built-in string join method, implement a join(arr, s) method, which receives an array of strings, arr, and a string, s, and returns a single string consisting of the strings in arr with s in between them.

Examples:

arr = ["join", "by", "space"], s = " "
- Output: "join by space"
arr = ["b", "", "k", "", "p", "r n", "", "d", "d"], s = "ee"
- Output: "beekeepeerneeeeddeed!!"

If strings in your language are immutable, assume that you have access to a function array_to_string(arr), which takes an array of individual characters and returns a string with those characters in $O(len(arr))$ time.

1def join(arr: List[str], s: str) -> str:

2 if not arr:

3 return ""

4 res = []

5 for i in range(len(arr)):

6 res.append(arr[i])

7 if i < len(arr) - 1:

8 res.append(s)

9 return "".join(res)

String Join

Reference solution