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Convert Roman Number to Decimal (Integer) | Write Python Program to Convert Roman to Integer

In this tutorial, we will write the Python program to convert the Roman numbers into the integer. It is a popular problem was asked by the tech giant Amazon, Facebook in the interview. Let's see the problem statement and implementation of the solution.

Problem Statement

A roman number is given as a string; the task is to convert the corresponding integer value. The symbols are given below for reference.

Symbols Values
I 1
IV 4
V 5
IX 9
X 10
XL 40
L 50
XC 90
C 100
CD 400
D 500
CM 900
M 1000

Example 1:

Input:s = VI

Output: 6

Example 2:

Input: XOutput: 10XL is a Roman symbol which represents 40

Solution Approach


  1. First, split the Roman numeral string into roman symbols.
  2. We can the separate symbol now convert each symbol of Roman Numerals into the value it represents.
  3. Picking up a value starting from index 0.
  • If the current value of the symbol is greater than or equal to the next symbol, then add this value to the returning total.
  • Else subtract this value by adding the value of the next symbol to the running total.

Let's implement the algorithm to the Python program.





In the above code, we define a rom_value() function which returns corresponding to the symbol. Next we define the romanTointeger() method which converts the value roman value to integer. In the romanToInteger() method,

  • We have defined the res and i variable with 0.
  • The while iterated till i is smaller than the length of the string.
  • We converted the first character into an integer and stored it into n1. Then, use the condition to check the i+1th element smaller than the length of the string.
  • If it returns true, it converts into an integer and is stored in n2.
  • Compare n1 and n2; if n1 is greater than the n2, add it to the res and increase the ith value by one.
  • If it returns false, then subtract the n2 to n1 and make the increment of 2 in i.
  • If first if condition returns false add into the res and make increment to i

Complexity Analysis

Time Complexity: O(n), where n is the length of the string. Only one traversal of the string is required.

Space Complexity: O(1). As no extra space is required

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