Description

Roman numerals are represented by seven different symbols: I , V , X , L , C , D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII , which is simply X + II . The number twenty seven is written as XXVII , which is XX + V + II .

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII . Instead, the number four is written as IV . Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

  • I can be placed before V (5) and X (10) to make 4 and 9.
  • X can be placed before L (50) and C (100) to make 40 and 90.
  • C can be placed before D (500) and M (1000) to make 400 and 900.

Given an integer, convert it to a roman numeral. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

Input: 3
Output: "III"

Example 2:

Input: 4
Output: "IV"

Example 3:

Input: 9
Output: "IX"

Example 4:

Input: 58
Output: "LVIII"
Explanation: C = 100, L = 50, XXX = 30 and III = 3.

Example 5:

Input: 1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

来源：leetcode 12 integer to roman

Solution

Approach 1

这道题并不难，方法也很容易想到，这道题被归为 medium 难度反而让我感到有些奇怪，下面的代码就是我的解法。

string intToRoman(int num) {

    // digit1 ~ 4 分别代表个十百千
    int digit1 = num%10;
    int digit2 = num/10%10;
    int digit3 = num/100%10;
    int digit4 = num/1000%10;

    string ans;

    // 千
    for (int i=0; i<digit4; ++i) {
	ans += 'M';
    }

    // 百
    switch (digit3) {
    case 1:
    case 2:
    case 3:
	for (int i=0; i<digit3; ++i) {
	    ans += 'C';
	}
	break;

    case 4:
	ans += "CD";
	break;

    case 5:
    case 6:
    case 7:
    case 8:
	ans += 'D';
	for (int i=5; i<digit3; ++i) {
	    ans += 'C';
	}
	break;

    case 9:
	ans += "CM";
	break;
    default:
	break;
    }

    // 十
    switch (digit2) {
    case 1:
    case 2:
    case 3:
	for (int i=0; i<digit2; ++i){
	    ans += 'X';
	}
	break;

    case 4:
	ans += "XL";
	break;

    case 5:
    case 6:
    case 7:
    case 8:
	ans += 'L';
	for (int i=5; i<digit2; ++i) {
	    ans += "X";
	}
	break;

    case 9:
	ans += "XC";
	break;

    default:
	break;
    }

    // 个
    switch (digit1) {
    case 1:
    case 2:
    case 3:
	for (int i=0; i<digit1; ++i) {
	    ans += 'I';
	}
	break;

    case 4:
	ans += "IV";
	break;

    case 5:
    case 6:
    case 7:
    case 8:
	ans += 'V';
	for (int i=5; i<digit1; ++i) {
	    ans += 'I';
	}
	break;

    case 9:
	ans += "IX";
	break;

    default:
	break;
    }

    return ans;
}

这样做感觉有点简单粗暴，而且代码看起来不够优雅。

Approach 2

下面代码是从leetcode上找的，这样的打表法，看起来至少从形式上会更加美观，而且也非常好理解。

class Solution {
public:
    const static string THOUS[];
    const static string HUNDS[];
    const static string TENS[];
    const static string ONES[];
    string intToRoman(int num) {
	string result;
	result += THOUS[(int)(num/1000)%10];
	result += HUNDS[(int)(num/100)%10];
	result += TENS[(int)(num/10)%10];
	result += ONES[num%10];
	return result;
    }
};

const string Solution::THOUS[]	= {"","M","MM","MMM"};
const string Solution::HUNDS[]	= {"","C","CC","CCC","CD","D","DC","DCC","DCCC","CM"};
const string Solution::TENS[]	= {"","X","XX","XXX","XL","L","LX","LXX","LXXX","XC"};
const string Solution::ONES[]	= {"","I","II","III","IV","V","VI","VII","VIII","IX"};

Approach 3

下面这段代码也是leetcode上的答案，这个答案的通用性要好一些，最起码找到了规律，这有助于算法的后续拓展。

class Solution {
public:
    string intToRoman(int num) {
	vector<int> base{1000, 100, 10, 1};
	string roman = "MDCLXVI";
	int index = 0, r;
	string ret = "";
	while(num) {
	    r = num/base[index];
	    num = num - r*base[index];
	    if(r == 4) {
		ret += roman[2*index];
		ret += roman[2*index-1];
	    }
	    else if(r == 9) {
		ret += roman[2*index];
		ret += roman[2*index -2];
	    }
	    else if(r >= 5) {
		ret += roman[2*index - 1];
		// 下面这个string的用法值得学习
		string tmp(r-5, roman[2*index]);
		ret += tmp;
	    }
	    else if(r < 5) {
		string tmp(r, roman[2*index]);
		ret += tmp;
	    }
	    index++;
	}
	return ret;
    }
};

此外，我还学到了 string tmp(num, ch) 这样的创建具有重复字符的字符串的方法。