Leetcode 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.

Example 1:

1 2	Input: "III" Output: 3

Example 2:

1 2	Input: "IV" Output: 4

Example 3:

1 2	Input: "IX" Output: 9

Example 4:

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2
3

Input: "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 5:

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2
3

Input: "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

一遍扫描

先构建一个字符到整数的映射, 扫描的时候碰到某个字符就加或减其对应的值

关键是如何确定加还是减

发现, 只需要比较第i位的字符和第i+1位的字符大小即可

例如, IX中, I < X, 所以I为-1而不是1.

MCMXCIV 中, 第一个C后面是M, 而C < M, 这时C就代表-100, 第二个C后面是I, C > I, 这时C就代表100.

class Solution {
public:
    int romanToInt(string s) {
        int ans = 0;
        for(int i = 0; i < s.size(); ++i)
        {
            if(i == s.size() - 1)
            {
                ans += charToInt[s[i]];
            }
            else
            {
                bool isNegative = (charToInt[s[i]] < charToInt[s[i+1]]);
                ans += isNegative ? -charToInt[s[i]] : charToInt[s[i]];
            }
        }
        return ans;
    }
    Solution()
    {
        init();
    }
private:
    unordered_map<char, int> charToInt;
    void init()
    {
        charToInt.insert({'I',1});
        charToInt.insert({'V',5});
        charToInt.insert({'X',10});
        charToInt.insert({'L',50});
        charToInt.insert({'C',100});
        charToInt.insert({'D',500});
        charToInt.insert({'M',1000});
    }
};