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Subsequence check

A subsequence of a string is a new string that is formed from the original string by removing zero or more characters without disturbing the relative positions of the remaining characters.

Ex: Original String = ABC
Subsequences = "", A, B, C, AB, AC, BC, ABC

Characteristics of a Subsequence:
1. Length of subsequence is always less than or equal to the original string.
2. Characters should always appear in the same order as in the original string.
3. The total number of subsequences of a string with length n will be 2nSince for every character we have 2 choices, either to keep it or remove it.

func checkIs(subsequence:String, from input:String)->Bool{
    guard subsequence.count <= input.count else { return false }
    
    var subsequenceIndex = subsequence.startIndex
    var inputIndex = subsequence.startIndex
    
    while inputIndex < input.endIndex &&
            subsequenceIndex < subsequence.endIndex {
        if subsequence[subsequenceIndex] == input[inputIndex]{
            subsequenceIndex = subsequence.index(after: subsequenceIndex)
        }
        inputIndex = input.index(after: inputIndex)
    }
    return subsequenceIndex == subsequence.endIndex
}
Time Complexity: O(n+m)
Space Complexity: O(1)

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