Leetcode #833: Find And Replace in String

func findReplaceString(_ S: String, _ indexes: [Int], _ sources: [String], _ targets: [String]) -> String {
    var answer = [Character]()
    var str = Array(S)
    var  index = 0
    var i = 0
    let offsets = indexes.enumerated().sorted(by: {$0.element < $1.element}).map {$0.offset}
    let indexes = offsets.map {indexes[$0]}
    let sources = offsets.map {sources[$0]}
    let targets = offsets.map {targets[$0]}
    
    while i < str.count {
        if index >= indexes.count {
            answer += Array(str[i...])
            break
        }
        if i == indexes[index] {
            if str.count - indexes[index] < sources[index].count {
                index += 1
                continue
            }
            var possible = true
            for j in stride(from: 0, to: sources[index].count, by: 1) {
                if str[i+j] != Array(sources[index])[j] {
                    possible = false
                    break
                }
            }
            if possible {
                answer.append(contentsOf:Array(targets[index]))
                i += sources[index].count
            }
            index += 1
        } else {
            answer.append(str[i])
            i += 1
        }
    }
    return String(answer)
}