Easy 013 Roman to Integer

Description:

将罗马字母的字符串转换为代表的整数
Roman numerals are usually written largest to smallest from left to right. However, 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.
Symbol       ValueI             1V             5X             10L             50C             100D             500M             1000

My Solution:

  1. 这题不难，用一个HashMap存罗马数字和具体数字的对应关系，然后遍历前后两两比较，该加加，该减减

    • 时间复杂度O(n)

Fast Solution:

    public static int romanToInt(String s) {        int num = 0;        int n = s.length();                for (int i = 0; i < n-1; i++) {            int curr = map(s.charAt(i));  //这里map是自己写的一个方法，里面用一个switch，相当于HashMap存对应            int next = map(s.charAt(i+1));            num = curr < next ? num - curr : num + curr;            //当时一直想着用一个temp来存减的值，所以没法用for就用了while&point指针，但其实就是很简单的比后面小的话在总值里面减去就可以，不需要temp        }            num += map(s.charAt(n-1));               return num;    }