13. Roman to Integer

Solution idea

My Solution - Two Pointers/Sliding Window of size 2

Case 1: If current pointed two roman letters can form a number, then add this number to result AND increment both left and right pointers by 2.

Case 2: O/w, if not, then add only the number that left points to AND increment both left and right pointers by 1.

Time complexity = $O(n)$ where $n$ is the length of input string.

Another Solution

Look at each single roman letter once at a time from back to front.

If the current letter's value < its next letter's value, they two can form a number. Otherwise, they can't.

Time complexity = $O(n)$ where $n$ is the length of input string.