12. Integer To Roman【medium】

解题思路：

这道题没有特别多技巧，直接按照转换规则进行转换即可，对每一位进行遍历，注意考虑4和9的特殊情况即可。

在查看Leetcode讨论区的时候还看到了一种脑洞大开的“全部映射法”，这里贴上来给大家欣赏一下，也开拓了一下思路，这种方法告诉我们在情况不多的时候，可以考虑一下这种思路，代码更为简洁。

实现代码：

// 12. Integer to Roman
string intToRoman(int num) {
  map<int, char> romanMap;
  romanMap[1] = 'I';
  romanMap[5] = 'V';
  romanMap[10] = 'X';
  romanMap[50] = 'L';
  romanMap[100] = 'C';
  romanMap[500] = 'D';
  romanMap[1000] = 'M';
  string res;
  for (int i = 3; i >= 0; i--) {
    int fold = pow(10, i);
    if (num / fold == 9) {
      res += romanMap[fold];
      res += romanMap[fold * 10];
    } else if (num / fold == 4) {
      res += romanMap[fold];
      res += romanMap[fold * 5];
    } else if (num / fold > 0) {
      if (num / fold >= 5) {
        res += romanMap[fold * 5];
        num -= 5 * fold;
      }
      for (int i = 0; i < num / fold; i++) {
        res += romanMap[fold];
      }
    }
    num %= fold;
  }
  return res;
}
// 脑洞大开的全部映射法
string intToRoman(int num) {
  string M[] = {"", "M", "MM", "MMM"};
  string C[] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
  string X[] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
  string I[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
  return M[num/1000] + C[(num%1000)/100] + X[(num%100)/10] + I[num%10];
}

问题描述：

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

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

For example, two is written asII in Roman numeral, just two one's added together. Twelve is written as,XII, which is simplyX+II. The number twenty seven is written asXXVII, which isXX+V+II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is notIIII. Instead, the number four is written asIV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written asIX. There are six instances where subtraction is used:

• Ican be placed beforeV(5) andX(10) to make 4 and 9.
• Xcan be placed beforeL(50) andC(100) to make 40 and 90.
• Ccan be placed beforeD(500) andM(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.