Given a roman numeral, convert it to an integer.
The answer is guaranteed to be within the range from 1 to 3999.
IV -> 4
XII -> 12
XXI -> 21
XCIX -> 99
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[LintCode] Roman to Integer 罗马数字转化成整数
原文：http://www.cnblogs.com/grandyang/p/5724219.html