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[record of question brushing] 13 Roman numeral to integer

2022-07-19 13:15:00 【InfoQ】

One 、 Title Description

source ：

Power button （LeetCode）

Roman numerals contain the following seven characters : I, V, X, L,C,D  and  M.

 character   The number 
I 1
V 5
X 10
L 50
C 100
D 500
M 1000

for example , Rome digital 2 Write to do  II , Two parallel 1 .12 Write to do  XII , That is to say  X + II . 27 Write to do   XXVII, That is to say  XX + V + II .

Usually , The small numbers in roman numbers are to the right of the big ones . But there are special cases , for example 4 Do not write  IIII, It is  IV. Numbers 1 In number 5 Left side , The number represented is equal to the large number 5 Decimal reduction 1 Value obtained 4 . similarly , Numbers 9 Expressed as  IX. This special rule only applies to the following six cases ：

I  Can be placed in  V (5)  and  X (10)  Left side , To express  4  and  9.
X  Can be placed in  L (50)  and  C (100)  Left side , To express  40  and  90. 
C  Can be placed in  D (500)  and  M (1000)  Left side , To express  400  and  900.

Given a Roman number , Convert it to an integer .

Example  1:

 Input : s = &quot;III&quot;
 Output : 3

Example  2:

 Input : s = &quot;IV&quot;
 Output : 4

Example  3:

 Input : s = &quot;IX&quot;
 Output : 9

Example  4:

 Input : s = &quot;LVIII&quot;
 Output : 58
 explain : L = 50, V= 5, III = 3.

Example  5:

 Input : s = &quot;MCMXCIV&quot;
 Output : 1994
 explain : M = 1000, CM = 900, XC = 90, IV = 4.

Tips ：

1 <= s.length <= 15

s Characters only ('I', 'V', 'X', 'L', 'C', 'D', 'M')

Topic data assurance s It's a valid Roman number , And it means that the integer is in the range [1, 3999] Inside

The test cases given in the title all conform to the Roman numeral writing rules , There will be no straddle, etc .

IL and IM Such an example does not meet the requirements of the title ,49 You should write XLIX,999 You should write CMXCIX .

Detailed rules for writing Roman numerals , You can refer to Rome digital -
Mathematics
.

Two 、 Thought analysis

Train of thought understanding and the previous question

【 Record of writing questions 】12. Integer to Roman number

similar , It's just a reverse transformation

Roman numerals by  
I,V,X,L,C,D,M
  constitute ;

When the small value is to the left of the large value , Then decrease the value , Such as  
IV=5-1=4
;

When the small value is to the right of the large value , Then add a small value , Such as  
VI=5+1=6
;
combined ： If the small value is placed to the left of the large value , Just subtract , Otherwise it's addition
.

3、 ... and 、 Code implementation

class Solution {
 Map<String, Integer> map = new HashMap<>() {{
 put(&quot;M&quot;, 1000);
 put(&quot;D&quot;, 500);
 put(&quot;C&quot;, 100);
 put(&quot;L&quot;, 50);
 put(&quot;X&quot;, 10);
 put(&quot;V&quot;, 5);
 put(&quot;I&quot;, 1);
 }};

 public int romanToInt(String s) {
 int sum = 0;
 int res = map.get(s.substring(0, 1));
 for (int i = 1; i < s.length(); i++) {
 int num = map.get(s.substring(i, i + 1));
 if (res < num) {
 sum -= res;
 } else {
 sum += res;
 }
 res = num;
 }
 sum += res;
 return sum;
 }
}