Although there are better ways to implement this kata, your algorithm should also work, as long as you fix two problems.

1. The data type of digits[i] is char, so adding it to int would add (usually)ASCII value of the character.
2. Throughout loops, the sum variable would only increase.

All numbers are equal except for one.

[1, 1, 3, 8, 8] is invalid input.

...very limited set of instructions, which are executed in an endless loop.

if DR is executed endlessly (i.e. DRDRDR...) it will fall into the hole at (5,5)
if DRD is executed endlessly, it will leave the area on the down side.

if X[i] > X[j] then X[i] = X[i] - X[j]

(6, 9, 9 - 9) = (6, 9, 0) step can't be done.

Apart from the case in which a ban phrase is a subset of another longer ban phrase, there should also be an explanation about the case in which a ban phrase isn't a subset of another ban phrase, yet share common words.

For example, what should be the answer for below situation?

banPhrases = ["as long", "long as"]
article = "as long as"

You should think about the case in which the user's rank is 8 and incProgress is called with level 9

You should know about how comparisons of strings work.
e.g) 10 > 2 but "10" < "2"

If "initialValue" is not specified, reduce function takes first element as "initialValue". Therefore, if there's a single element in the list, your function would just return first element of the list, which might not be number.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax

I guess you are not familiar with c.
You should figure out how to compare strings in c.

I guess you are joining strings by String operations. (e.g. output += "1")
Try using StringBuffers instead.

...
} else if (finalWords.contains("7")) {
arrOfWords[6] = finalWords;
}
...

Above is the wrong part. I guess you can easily fix it by yourself :)