13. Roman to Integer

Problem Statement

Pattern:

---

Solution

public int value (char ch) {
	switch (ch) {
		case 'I' : return 1;
		case 'V' : return 5;
		case 'X' : return 10;
		case 'L' : return 50;
		case 'C' : return 100;
		case 'D' : return 500;
		default : return 1000;
	}
}
 
public int romanToInt(String s) {
	int prev = -1, sum = 0;
	for (char ch : s.toCharArray()) {
		int curr = value(ch);
		sum+= curr;
		
		if(prev != -1 && prev < curr ) sum -= 2*prev;
		prev = curr;
	}
	return sum;
}

TC : $O(n)$ SC : $O(1)$

Notes

• if a smaller number comes before a larger number, instead of adding it to the sum, we subtract it from the sum.