We iterate through the string and add the value of each symbol to the total. If a smaller symbol appears before a larger one, we subtract it instead of adding it.

1. Map characters I, V, X, L, C, D, M to values.
2. Initialize total = 0, prev_val = 0.
3. Iterate through characters in reverse:
   • If curr_val >= prev_val, total += curr_val.
   • Else, total -= curr_val.
   • Update prev_val.
4. Return total.

• Time Complexity: O(N).
• Space Complexity: O(1).

def roman_to_int(s):
    d = {'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
    res = 0
    prev_val = 0
    
    for char in reversed(s):
        curr_val = d[char]
        if curr_val >= prev_val:
            res += curr_val
        else:
            res -= curr_val
        prev_val = curr_val
        
    return res