Create 12. Integer to Roman.md

12. Integer to Roman.md

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+# [12. Integer to Roman](https://leetcode.com/problems/integer-to-roman/)
+# 思路
+阿拉伯数字转罗马数字。
+按照题意将所有可能的罗马数字列出来，再从大到小看能不能将阿拉伯数字转换成罗马数字。详细过程见代码。
+
+# C++
+``` C++
+class Solution {
+public:
+    string intToRoman(int num) {
+        map<int, string>int2rom;
+        string res = "";
+        int nums[13] = {1,4,5,9,10,40,50,90,100,400,500,900,1000};
+        int2rom[1] = "I";
+        int2rom[5] = "V";
+        int2rom[10] = "X";
+        int2rom[50] = "L";
+        int2rom[100] = "C";
+        int2rom[500] = "D";
+        int2rom[1000] = "M";
+        int2rom[4] = "IV";
+        int2rom[9] = "IX";
+        int2rom[40] = "XL";
+        int2rom[90] = "XC";
+        int2rom[400] = "CD";
+        int2rom[900] = "CM";
+        for(int i = 12; i >= 0; i--){
+            int tmp = num / nums[i];
+            while(tmp--) res += int2rom[nums[i]];
+            num %= nums[i];
+        }
+        return res;
+    }
+};
+```