13. Roman to Integer

LeetCode 13. Roman to Integer

Description

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, two is written as II in Roman numeral, just two one’s added together. Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

• I can be placed before V (5) and X (10) to make 4 and 9.

• X can be placed before L (50) and C (100) to make 40 and 90.

• C can be placed before D (500) and M (1000) to make 400 and 900.

Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

Tags

Math, String

Solution

遍历字符串，先检查连续的两个字符是否存在映射，如果存在则将相应的数字累加在结果上，如果不存在或者已经遍历到最后一个字符，则将当前这一个字符映射的数字累加在结果上，最后返回结果。

Before start, collate all mapping from symbol to value. Traverse the string and check if there is a mapping for the current 2 consecutive characters first. If it exists, the corresponding number is added to the result. Otherwise the single-character symbol corresponded value is accumulated on the result.

Complexity

• Time complexity: $O(1)$

• Space complexity: $O(1)$

Code

func romanToInt(s string) int {
	var ans int
	for i := 0; i < len(s); i++ {
		var singleFlag bool
		if i+1 < len(s) {
			switch s[i : i+2] {
			case "IV":
				ans += 4
				i++
			case "IX":
				ans += 9
				i++
			case "XL":
				ans += 40
				i++
			case "XC":
				ans += 90
				i++
			case "CD":
				ans += 400
				i++
			case "CM":
				ans += 900
				i++
			default:
				singleFlag = true
			}
		} else {
			singleFlag = true
		}
		if singleFlag {
			switch s[i] {
			case 'I':
				ans += 1
			case 'V':
				ans += 5
			case 'X':
				ans += 10
			case 'L':
				ans += 50
			case 'C':
				ans += 100
			case 'D':
				ans += 500
			case 'M':
				ans += 1000
			}
		}
	}
	return ans
}