14. 最长公共前缀

编写一个函数来查找字符串数组中的最长公共前缀。
如果不存在公共前缀，返回空字符串 “”。

方法一 竖向扫描法

个人感觉纵向扫描方式比较直观，符合人类理解方式，从前往后遍历所有字符串的每一列，比较相同列上的字符是否相同，如果相同则继续对下一列进行比较，如果不相同则当前列不再属于公共前缀，当前列之前的部分为最长公共前缀。

Swift

func longestCommonPrefix(_ strs: [String]) -> String {
        guard let firstStr = strs.first, !firstStr.isEmpty else { return "" }
        
        for i in 0..<firstStr.count {
            for j in 1..<strs.count {
                if strs[j].count == i || strs[j][strs[j].index(strs[j].startIndex, offsetBy: i)] != firstStr[firstStr.index(firstStr.startIndex, offsetBy: i)] {
                    return String(firstStr.prefix(i))
                }
            }
        }
        
        return firstStr
    }

OC

-(NSString *)longestCommonPrefix:(NSArray <NSString *>*)strs {
    if (strs.count <= 0) {
        return @"";
    }
    
    NSString *firstStr = strs.firstObject;
    NSInteger len = firstStr.length;
    for (NSInteger i=0; i<len; i++) {
        for (NSInteger j=1; j<strs.count; j++) {
            if (strs[j].length == i || [strs[j] characterAtIndex:i] != [firstStr characterAtIndex:i]) {
                return [firstStr substringToIndex:i];
            }
        }
    }
    
    return firstStr;
}

方法二 有序首尾比较法

有序首尾比较法，先对数组进行排序，巧妙利用排序后的顺序及值之间的关系，只比较首尾两个字符串即可。

Swift

func longestCommonPrefix(_ strs: [String]) -> String {
        let strs = strs.sorted()
        
        let start = strs.first!
        let end = strs.last!
        
        var res = ""
        for i in 0..<start.count {
            let s = start[start.index(start.startIndex, offsetBy: i)]
            if s == end[end.index(end.startIndex, offsetBy: i)]{
                res.append(s)
            }else {
                break
            }
        }
        return res
    }

OC

//有序首尾比较法
-(NSString *)longestCommonPrefix:(NSArray *)strs {
    NSArray *sortedStrs = [strs sortedArrayUsingComparator:^NSComparisonResult(NSString *obj1, id  _Nonnull obj2) {
        return [obj1 compare:obj2 options:NSCaseInsensitiveSearch];
    }];
    
    NSString *res = @"";
    NSString *firstStr = sortedStrs.firstObject;
    NSString *lastStr = sortedStrs.lastObject;
    for (NSInteger i=0; i<firstStr.length; i++) {
        if ([firstStr characterAtIndex:i] == [lastStr characterAtIndex:i]) {
            unichar c = [firstStr characterAtIndex:i];
            res = [res stringByAppendingString:[NSString stringWithCharacters:&c length:1]];
        }else {
            break;
        }
    }
    
    return res;
}