# [LeetCode-13]-Roman to Integer (将罗马数字转换为整数)【附源码】_donaldsy

## 题目相关

【题目解读】

【原题描述】原题链接
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 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: L = 50, V= 5, III = 3.  Example 5: Input: "MCMXCIV" Output: 1994 Explanation: M = 1000, CM = 900, XC = 90 and IV = 4. ``

【难度】Easy

## Solution

``    int romanToInt(string s)     {         int ret = 0;         for (size_t i = 0; i < s.size(); i++)         {             /* code */             switch (s[i])             {                 //I = 1; V = 5; X = 10; L = 50; C = 100; D = 500; M = 1000             case 'I':                 ret += 1;                 break;                  //I = 1; V = 5; X = 10; L = 50; C = 100; D = 500; M = 1000             case 'V':                 ret += 5;                  if (s[i - 1] == 'I')                 {                     //IV的这种情况需要恢复                     ret -= 2;                 }                 break;                 //I = 1; V = 5; X = 10; L = 50; C = 100; D = 500; M = 1000             case 'X':                 ret += 10;                 if (s[i - 1] == 'I')                 {                     //IX的这种情况需要恢复                     ret -= 2;                 }                 break;                  //I = 1; V = 5; X = 10; L = 50; C = 100; D = 500; M = 1000             case 'L':                 ret += 50;                 if (s[i - 1] == 'X')                 {                     //XL的这种情况需要恢复                     ret -= 20;                 }                 break;                  //I = 1; V = 5; X = 10; L = 50; C = 100; D = 500; M = 1000             case 'C':                 ret += 100;                 if (s[i - 1] == 'X')                 {                     //XC的这种情况需要恢复                     ret -= 20;                 }                 break;                  //I = 1; V = 5; X = 10; L = 50; C = 100; D = 500; M = 1000             case 'D':                 ret += 500;                 if (s[i - 1] == 'C')                 {                     //CD的这种情况需要恢复                     ret -= 200;                 }                 break;                  //I = 1; V = 5; X = 10; L = 50; C = 100; D = 500; M = 1000             case 'M':                 ret += 1000;                 if (s[i - 1] == 'C')                 {                     //CM的这种情况需要恢复                     ret -= 200;                 }                 break;              default:                 return 0;             }//end switch         }//end for          return ret;     } ``

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