14.最长公共前缀

https://leetcode-cn.com/problems/longest-common-prefix/

func longestCommonPrefix(_ strs: [String]) -> String {
    if strs.count <= 0 {return ""}
    var preStr = strs[0]
    for (index, str) in strs.enumerated() {
        if index == 0 {continue}
        while !str.hasPrefix(preStr) {
            let startIndex = preStr.index(preStr.startIndex, offsetBy: 0)
            let endIndex = preStr.index(preStr.startIndex, offsetBy: (preStr.count - 1))
            //每次删除末尾一个字符
            let tmpStr = preStr[startIndex..<endIndex]
            preStr = String(tmpStr)
            if preStr.count == 0 {return ""}
        }
    }
    return preStr
}