given a string, find its longest substring that contains two unique characters

longest2Csubstring.swift

let str = "abcbbbbcccbdddadacb"

var maxLength = 0
var startForMax = 0

// recurse
// complexity: O(N)
func search(start: Int) {
    let i = start
    var j = start + 1

    let a = str[str.index(str.startIndex, offsetBy: i)]
    var b = str[str.index(str.startIndex, offsetBy: j)]
    while j < str.count && b == a {
        j += 1
        b = str[str.index(str.startIndex, offsetBy: j)]
    }

    while j < str.count && [a, b].contains(str[str.index(str.startIndex, offsetBy: j)]) {
        j += 1
    }

    if j - i > maxLength {
        maxLength = j - i
        startForMax = i
    }

    if j < str.count - 1 {
        search(start: i + 1)
    }
}

// start from 0
search(start: 0)

// get the result
let substr = String(str[str.index(str.startIndex, offsetBy: startForMax)...str.index(str.startIndex, offsetBy: startForMax + maxLength - 1)])
print(substr)