Roman numerals contain the following seven characters: I, V, X, L, C, D and M.
字符 数值
I 1
V 5
X 10
L 50
C 100
D 500
M 1000
For example, the Roman numeral 2 is written as II, which means two parallel ones. 12 is written as XII, which means X + II. 27 is written as XXVII, which is XX + V + II. Normally, the small numbers in Roman numerals are to the right of the large numbers. But there are special cases, for example, 4 is not written as IIII, but IV. The number 1 is to the left of the number 5, and the number represented is equal to the number 4 obtained by subtracting the number 1 from the large number 5. Similarly, the number 9 is represented as IX. This special rule only applies to the following six cases: I can be placed to the left of V (5) and X (10) to represent 4 and 9. X can be placed to the left of L (50) and C (100) to represent 40 and 90. C can be placed to the left of D (500) and M (1000) to represent 400 and 900.
Given a Roman numeral, convert it to an integer. Ensure that the input is in the range of 1 to 3999.
answer:
罗马数字由 I,V,X,L,C,D,M 构成；
当一个小值在大值的左边，则减小值，如 IV=5-1=4；
当一个小值在大值的右边，则加小值，如 VI=5+1=6；
由上可知，右值永远为正，因此最后一位必然为正。
Since I have not yet come into contact with the study of Java data structures, map is not used, but a method is defined to reflect the integer represented by each character.
public int getValue(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;
case 'M': return 1000;
default: return 0;
}
}
The for loop compares each character with the integer represented by the character to the right to add or subtract.
class Solution {
public int romanToInt(String s) {
int sum = 0;
int preNum = getValue(s.charAt(0));
for(int i = 1;i < s.length(); i ++) {
int num = getValue(s.charAt(i));
if(preNum < num) {
sum -= preNum;
} else {
sum += preNum;
}
preNum = num;
}
sum += preNum;
return sum;
}
public int getValue(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;
case 'M': return 1000;
default: return 0;
}
}
}