LeetCode //13. Roman to Integer

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 ones 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.

 

Example 1:

Input: s = “III”
Output: 3
Explanation: III = 3.

Example 2:

Input: s = “LVIII”
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 3:

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

Constraints:

• 1 <= s.length <= 15
• s contains only the characters (‘I’, ‘V’, ‘X’, ‘L’, ‘C’, ‘D’, ‘M’).
• It is guaranteed that s is a valid roman numeral in the range [1, 3999].

From: LeetCode
Link: 13. Roman to Integer

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Solution:

Ideas：

This program first defines a helper function value() to convert a single Roman numeral character to its integer value. Then, in the romanToInt() function, it processes the input string one character at a time. If it encounters a case where a smaller numeral appears before a larger one (which is the subtractive case), it subtracts the value of the smaller numeral. Otherwise, it adds the value of the numeral. Finally, the main() function gets the input from the user and calls the romanToInt() function to convert the input to an integer.

Code:

int value(char roman) {
    switch (roman) {
        case 'I': return 1;
        case 'V': return 5;
        case 'X': return 10;
        case 'L': return 50;
        case 'C': return 100;
        case 'D': return 500;
        case 'M': return 1000;
        default: return 0;
    }
}

int romanToInt(char *s) {
    int total = 0;
    while(*s) {
        if(*s == '\0') {
            break;
        }

        // If this is the subtractive case.
        if(value(*s) < value(*(s+1))) {
            total -= value(*s);
        } else {
            total += value(*s);
        }

        // Move on to the next character.
        s++;
    }

    return total;
}