13. Roman To Integer【easy】

解题思路：

这道题和上一题一样，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。

实现代码：

// 13. Roman To Integer
int romanToInt (string s) {
  int res = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == 'I') {
      res += 1;
    } else if (s[i] == 'V') {
      res += 5;
      if (i - 1 >= 0 && s[i - 1] == 'I') {
        res -= 2;
      }
    } else if (s[i] == 'X') {
      res += 10;
      if (i - 1 >= 0 && s[i - 1] == 'I') {
        res -= 2;
      }
    } else if (s[i] == 'L') {
      res += 50;
      if (i - 1 >= 0 && s[i - 1] == 'X') {
        res -= 20;
      }
    } else if (s[i] == 'C') {
      res += 100;
      if (i - 1 >= 0 && s[i - 1] == 'X') {
        res -= 20;
      }
    } else if (s[i] == 'D') {
      res += 500;
      if (i - 1 >= 0 && s[i - 1] == 'C') {
        res -= 200;
      }
    } else if (s[i] == 'M') {
      res += 1000;
      if (i - 1 >= 0 && s[i - 1] == 'C') {
        res -= 200;
      }
    }
  }
  return res;
}

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

Example 1:

Input: "III"
Output: 3

Example 2:

Input: "IV"
Output: 4

Example 3:

Input: "IX"
Output: 9

Example 4:

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

Example 5:

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