Rust: codewars的Roman Numerals Encoder

算法很简单，其实就是输入一个阿拉伯数字，输出一个罗马数字。
说明：但是这个只是一个简单的版本，输入的数字不算大，不涉及相对较大数的复杂罗马数字表达（比如，上面带横线之类）。
比如：
1990 =>:1000=M, 900=CM, 90=XC; => MCMXC.
2008 => 2000=MM, 8=VIII; => MMVIII.
1666 => MDCLXVI.
1、一些提示
Symbol Value
I 1
V 5
X 10
L 50
C 100
D 500
M 1,000

2、同一个字符，最多可以重复3次。
3、40，400的可以分别用XL,CD来表达。
其它的说明，可以找度娘。

一、我的算法

fn num_as_roman(num: i32) -> String {let data: HashMap<i32, &str> = [(0, ""),(1, "I"),(2, "II"),(3, "III"),(4, "IV"),(5, "V"),(6, "VI"),(7, "VII"),(8, "VIII"),(9, "IX"),(10, "X"),(20, "XX"),(30, "XXX"),(40, "XL"),(50, "L"),(60, "LX"),(70, "LXX"),(80, "LXXX"),(90, "XC"),(100, "C"),(200, "CC"),(300, "CCC"),(400, "CD"),(500, "D"),(600, "DC"),(700, "DCC"),(800, "DCCC"),(900, "CM"),(1000, "M"),(2000, "MM"),(3000, "MMM")].into_iter().map(|w| (w.0, w.1)).collect::<HashMap<i32, &str>>();let chars: Vec<char> = num.to_string().chars().into_iter().map(|x| x).collect();let fact_nums = fact_num(num);fact_nums.into_iter().map(|x| *(data.get(&(x.0 * x.1)).unwrap())).collect::<Vec<&str>>().join("")
}
fn fact_num(num: i32) -> Vec<(i32, i32)> {let len = num.to_string().len() as i32;let mut result: Vec<(i32, i32)> = Vec::new();let mut remain: i32 = num;for i in 0..len {let k = len - i - 1;let pow_value = 10_f64.powf(k as f64) as i32;let temp = remain / pow_value;remain = remain - temp * pow_value;if k < 1 {result.push((1, temp));} else {result.push((temp, pow_value));}println!("temp :{:?}, remain:{:?}", temp, remain);}result
}

当然，也可以用 HashMap

fn num_as_roman(num: i32) -> String {let data: HashMap<i32, String> = [(0, ""),(1, "I"),(2, "II"),(3, "III"),(4, "IV"),(5, "V"),(6, "VI"),(7, "VII"),(8, "VIII"),(9, "IX"),(10, "X"),(20, "XX"),(30, "XXX"),(40, "XL"),(50, "L"),(60, "LX"),(70, "LXX"),(80, "LXXX"),(90, "XC"),(100, "C"),(200, "CC"),(300, "CCC"),(400, "CD"),(500, "D"),(600, "DC"),(700, "DCC"),(800, "DCCC"),(900, "CM"),(1000, "M"),(2000, "MM"),(3000, "MMM")].into_iter().map(|w| (w.0, w.1.to_string())).collect::<HashMap<i32, String>>();let chars: Vec<char> = num.to_string().chars().into_iter().map(|x| x).collect();let fact_nums = fact_num(num);fact_nums.into_iter().map(|x| (&data).get(&(x.0 * x.1)).unwrap().to_string()).collect::<Vec<String>>().join("")
}

二、精彩的算法

fn rep(c: char, n: usize) -> String {std::iter::repeat(c).take(n).collect()
}fn roman(d: u8, p: usize) -> String {let nines = ["IX", "XC", "CM"];let fives = ["V", "L", "D"];let fours = ["IV", "XL", "CD"];let ones  = ['I', 'X', 'C', 'M'];match d {0 => String::from(""),9 => String::from(nines[p]),n if n > 5 => String::from(fives[p]) + &rep(ones[p], (n - 5) as usize),5 => String::from(fives[p]),4 => String::from(fours[p]),n => rep(ones[p], n as usize),}
}fn num_as_roman(num: i32) -> String {let s = num.to_string();let l = s.len();let mut r = String::new();for (i, d) in s.chars().enumerate() {let p = (l - i - 1) as usize;r += &roman(d.to_digit(10).unwrap() as u8, p);}r
}

2、

const ONES: [&'static str; 10] = ["", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"];
const TENS: [&'static str; 10] = ["", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"];
const HUNS: [&'static str; 10] = ["", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"];
const TOUS: [&'static str; 10] = ["", "M", "MM", "MMM", "", "", "", "", "", ""];
const DIGITS: [[&'static str; 10]; 4] = [ONES, TENS, HUNS, TOUS];fn num_as_roman(num: i32) -> String {[num/1000, num/100, num/10, num].into_iter().enumerate().map(|(i, &d)| DIGITS[3 - i][(d % 10) as usize]).collect()
}

3、

static ROMAN_DEC: [(&'static str, i32); 13] = [("M", 1000), ("CM", 900), ("D", 500), ("CD", 400),("C",  100), ("XC",  90), ("L",  50), ("XL",  40),("X",   10), ("IX",   9), ("V",   5), ("IV",   4),("I",    1)];/// Converts a number to a string representating roman numeral.
fn num_as_roman(num: i32) -> String {let mut num = num;let mut result = String::new();for &(ref lit, val) in &ROMAN_DEC {while num >= val {result.push_str(lit);num -= val;}if num == 0 { break }}result
}

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