LeetCode知识点总结 - 13

LeetCode 13. Roman to Integer

考点	难度
Hash Table	Easy

题目

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.

思路

用map储存罗马字母和对应的数字。如果s中的罗马字母对应数字比前一位小，减去前一位数字的两倍（因为上一轮已经在总数上加过前一位数字）。

答案

public int romanToInt(String s) {
        int res = 0;
        HashMap<Character,Integer> map = new HashMap<Character,Integer>();
        map.put('I',1);
        map.put('V',5);
        map.put('X',10);
        map.put('L',50);
        map.put('C',100);
        map.put('D',500);
        map.put('M',1000);
        res += map.get(s.charAt(0));
        for(int i=1;i<s.length();i++){
            int cur = map.get(s.charAt(i));
            int pre = map.get(s.charAt(i-1));
            if (pre<cur) res -= pre*2;
            res += cur;
        }
        return res;
    }