14. 最长公共前缀 LeetCode

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class Solution {
    func longestCommonPrefix(_ strs: [String]) -> String {
        if strs.count < 1 {
            return "";
        }else{
            var result = strs.first;
            for i in 1..<strs.count{
                var count = 0;
                if (result?.count)! < strs[i].count{
                    count = (result?.count)!
                }else{
                    count = strs[i].count
                }
                if count == 0{
                    return ""
                }
                for j in 0..<count{
                    let c = result?.charAt(index: j);
                    let c2 = strs[i].charAt(index: j);
                    if c == c2{
                        if j == count - 1{
                            result = String((result?.prefix(j + 1))!);
                        }
                    }else{
                        result = String((result?.prefix(j))!);
                    }
                    if result?.count == 0 {
                        return result!;
                    }
                }
            }
            return result!;
        }
    }
}
extension String{
    func charAt(index:Int)->Character?{
        if index >= self.count || index < 0{
            return nil
        }
        let i = self.index(self.startIndex, offsetBy:index)
        return self[i]
    }
}