LeetCode 013 Roman to Integer

LeetCode

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, 2 is written as II in Roman numeral, just two one’s added together. 12 is written as XII, which is simply X + II. The number 27 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.

Example 1:

Input: s = “III”
Output: 3
Explanation: III = 3.

Example 2:

Input: s = “LVIII”
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 3:

Input: s = “MCMXCIV”
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

Constraints: 1 <= s.length <= 15
s contains only the characters ('I', 'V', 'X', 'L', 'C', 'D', 'M'). It is guaranteed that s is a valid roman numeral in the range [1, 3999].

题目大意：

罗马数组转阿拉伯数字

解题思路：

按照规则累加。有一个特别规则是需要做减法如IV。

解题步骤：

N/A

注意事项：

1. 先加再减的方法。
2. SYMBOL_TO_VAL的值可以哟用于判断先后顺序。

Python代码：

def romanToInt(self, s: str) -> int:
	SYMBOL_TO_VAL = {'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
	res, num, prev = 0, 0, ''
	for symbol in s:
		num = SYMBOL_TO_VAL[symbol]
		if prev and SYMBOL_TO_VAL[prev] < SYMBOL_TO_VAL[symbol]:
			res -= SYMBOL_TO_VAL[prev] * 2
		res += num
		prev = symbol
	return res

算法分析：

时间复杂度为O(n)，空间复杂度O(1)