13. Roman to Integer

Math

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
class Solution {
    public int romanToInt(String s) {
        int res = 0;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            char nextC = i + 1 < s.length() ? s.charAt(i + 1) : ' ';
            if ((c == 'I' && (nextC == 'V' || nextC == 'X')) || (c == 'X' && (nextC == 'L' || nextC == 'C')) || (c == 'C' && (nextC == 'D' || nextC == 'M'))) {
                res += romanUnitToInt("" + c + nextC);
                i++; // 不要忘记偏移 i, 因为消耗了两个字符
            } else {
                res += romanUnitToInt(String.valueOf(c));
            } 
        }

        return res;
    }

    private int romanUnitToInt(String s) {
        switch (s) {
            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;
            case "IV":
                return 4;
            case "IX":
                return 9;
            case "XL":
                return 40;
            case "XC":
                return 90;
            case "CD":
                return 400;
            case "CM":
                return 900;
            default:
                throw new IllegalArgumentException("Illegal raman unit: " + s);
        }
    }
}

Math

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
class Solution {
    public int romanToInt(String s) {
        int res = 0;
        int preNum = romanUnitToInt(s.charAt(0));
        for (int i = 1; i < s.length(); i++) {
            char c = s.charAt(i);
            int num = romanUnitToInt(c);
            if (preNum < num) {
                res -= preNum;
            } else {
                res += preNum;
            }
            preNum = num;
        }
        res += preNum;

        return res;
    }

    private int romanUnitToInt(char s) {
        switch (s) {
            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:
                throw new IllegalArgumentException("Illegal roman char: " + s);
        }
    }
}

References

13. Roman to Integer