描述
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.
Example 1:
Input: "III"
Output: 3
Example 2:
Input: "IV"
Output: 4
Example 3:
Input: "IX"
Output: 9
Example 4:
Input: "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.
Example 5:
Input: "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
解析
根据题意，就是在计算罗马字符串转为十进制数，需要特别注意的是 'IV','IX','XL','XC','CD','CM' 这几个组合的特殊情况，剩下的直接遍历相加即可，时间复杂度为 O(N)，空间复杂度为 O(1)。
解答
class Solution(object):
def romanToInt(self, s):
"""
:type s: str
:rtype: int
"""
d = {'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000,'IV':4,'IX':9,'XL':40,'XC':90,'CD':400,'CM':900}
res = 0
i = 0
while i<len(s):
if s[i:i+2] in d:
res+=d[s[i:i+2]]
i+=2
continue
elif s[i:i+1] in d:
res+=d[s[i:i+1]]
i+=1
return res
运行结果
Runtime: 36 ms, faster than 82.04% of Python online submissions for Roman to Integer.
Memory Usage: 11.4 MB, less than 100.00% of Python online submissions for Roman to Integer.
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