LeetCode Daily Question 7

3 月 12 日 每日一题

题目：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, 2 is written as II in Roman numeral, just two one’s added together. 12 is written as XII, which is simply X + II. The number 27 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.

eg:

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

思路

使用 hashmap 模拟字典，从前向后遍历 string （除最后一位），如果当前的 vlue 比下一 value 下，说明为 IV 等特殊形式，此时减去此 value，否则加上此 value，for 循环结束后加上最后一个值。

代码

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class Solution {
public:
    int romanToInt(string s) {
        unordered_map<char, int> roman = { { 'I' , 1 },
                                   { 'V' , 5 },
                                   { 'X' , 10 },
                                   { 'L' , 50 },
                                   { 'C' , 100 },
                                   { 'D' , 500 },
                                   { 'M' , 1000 } };
        int res = 0;
        for(int i = 0; i < s.length() - 1; i++){
            if(roman[s[i]] < roman[s[i + 1]])
                res -= roman[s[i]];
            else
                res += roman[s[i]];
        }
        res += roman[s.back()];
        return res;
    }
};