13. Roman to Integer

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.

分析：题目的意思是把罗马数字转会换成阿拉伯数字，一般的排列顺序是由“大”到“小”，如VI代表6；一旦遇到了逆序，如IV，那么此时代表5-1=4。
思路：遍历整个字符串，如果当前字符小于下一个字符，就遇到了逆序，那么需要总值减去当前字符值，否则加上当前值。
代码：

func romanToInt(_ s: String) -> Int {
        var result  = 0;
        
        let signalValues:[String:Int] = ["I":1,"V":5,"X":10,"L":50,"C":100,"D":500,"M":1000];
        let chars = Array(s)
        
        for i in 0..<chars.count {
            let first = signalValues[String(chars[i])]!
            if i < chars.count-1 && first < signalValues[String(chars[i+1])]! {
                result -= first
                continue
            }
            result += first
        }
        return result

    }