Leetcode #1048: Longest String Chain

func longestStrChain(_ words: [String]) -> Int {
    let words = words.sorted { $0.count < $1.count }
    var dp = Array(repeating: 1, count: words.count)
    var answer = 1
    for i in stride(from: 1, to: words.count, by: 1) {
        for j in stride(from: i-1, through: 0, by: -1) {
            let diff = words[i].count - words[j].count
            if diff == 0 {
                continue
            } else if diff > 1 {
                break
            } else {
                if isFront(words[j], words[i]) {
                    dp[i] = max(dp[i], dp[j]+1)
                }
                answer = max(answer, dp[i])
            }
        }
    }
    return answer
}

private func isFront(_ previous: String, _ next: String) -> Bool {
    var i = 0
    var j = 0
    var previous = Array(previous)
    var next = Array(next)
    while i < previous.count {
        if previous[i] == next[j] {
            i += 1
            j += 1
        } else {
            j += 1
            if j - i > 1 {
                return false
            }
        }
    }
    return true
}