docs: 添加_13_romanToInt

Author: wangpeng

README.md

@@ -13,7 +13,7 @@
 共9道题目，4道Easy，5道Medium
 
 - [x] [12. 整数转罗马数字](https://leetcode-cn.com/problems/integer-to-roman)
-- [ ] [13. 罗马数字转整数](https://leetcode-cn.com/problems/roman-to-integer)
+- [x] [13. 罗马数字转整数](https://leetcode-cn.com/problems/roman-to-integer)
 - [ ] [16. 最接近的三数之和](https://leetcode-cn.com/problems/3sum-closest)
 - [ ] [17. 电话号码的字母组合](https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number)
 - [ ] [18. 四数之和](https://leetcode-cn.com/problems/4sum)
@@ -70,7 +70,8 @@
 | #9    | [回文数](https://leetcode-cn.com/problems/palindrome-number/) | [IsPalindrome](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_9_isPalindrome.java) | [数学](<https://leetcode-cn.com/tag/math/>)                  | Easy   |                                                              |
 | #10   | [正则表达式匹配](https://leetcode-cn.com/problems/regular-expression-matching/) | [IsMatch](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_10_isMatch_2.java) | [字符串](<https://leetcode-cn.com/tag/string/>)、[动态规划](<https://leetcode-cn.com/tag/dynamic-programming/>)、[回溯算法](<https://leetcode-cn.com/tag/backtracking/>) | Hard   | [自己原始解法](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_10_isMatch.java) |
 | #11   | [盛最多水的容器](https://leetcode-cn.com/problems/container-with-most-water/) | [MaxArea](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_11_maxArea.java) | [数组](<https://leetcode-cn.com/tag/array/>)、[双指针](<https://leetcode-cn.com/tag/two-pointers/>) | Medium |                                                              |
-| #12   | [整数转罗马数字](https://leetcode-cn.com/problems/integer-to-roman/) | [IntToRoman](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_12_intToRoman.java) | [数学](<https://leetcode-cn.com/tag/math/>)、[字符串](<https://leetcode-cn.com/tag/string/>) | Medium |                                                              |
+| #12   | [整数转罗马数字](https://leetcode-cn.com/problems/integer-to-roman/) | [IntToRoman](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_12_intToRoman.java) | [数学](<https://leetcode-cn.com/tag/math/>)、[字符串](<https://leetcode-cn.com/tag/string/>) | Medium | 反向求解#13                                                  |
+| #13   | [罗马数字转整数](https://leetcode-cn.com/problems/roman-to-integer/) | [RomanToInt](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_13_romanToInt.java) | [数学](<https://leetcode-cn.com/tag/math/>)、[字符串](<https://leetcode-cn.com/tag/string/>) | Easy   | 反向求解#12                                                  |
 | #14   | [最长公共前缀](https://leetcode-cn.com/problems/longest-common-prefix/) | [LongestCommonPrefix](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_14_longestCommonPrefix.java) | [字符串](<https://leetcode-cn.com/tag/string/>)              | Easy   |                                                              |
 | #15   | [三数之和](https://leetcode-cn.com/problems/3sum/)           | [ThreeSum](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_15_threeSum.java) | [数组](<https://leetcode-cn.com/tag/array/>)、[双指针](<https://leetcode-cn.com/tag/two-pointers/>) | Medium |                                                              |
 | #19   | [删除链表的倒数第N个节点](https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/) | [RemoveNthFromEnd](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_19_RemoveNthFromEnd.java) | [链表](https://leetcode-cn.com/tag/linked-list/)、[双指针](<https://leetcode-cn.com/tag/two-pointers/>) | Medium |                                                              |