Roman to integer

Link: https://leetcode.com/problems/roman-to-integer/

Solution:

Topics: math

Intuition
This problem is quite simple once you wrap your head around the logic. Basically, each letter in the roman numeral maps to a positive value UNLESS its followed by a larger letter…in that case the contribution of the letter is negative.

The last letter can never be negative because nothing follows it.

Implementation

def roman_to_integer(s):
	mapping = {
		'I': 1,
		'V': 5,
		'X': 10,
		'L': 50,
		'C': 100,
		'D': 500,
		'M': 1000
	}
	val = 0
	for i in range(len(s)-1):
		if i == len(s)-1 or mapping[s[i]] >= mapping[s[i+1]]:
			val += mapping[s[i]]
		else:
			val -= mapping[s[i]]
	return val
 
#time: o(n)
#memory: o(1)

Visual

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