【LeetCode Easy】013 Roman to Integer

Easy 013 Roman to Integer

Description:

將羅馬字母的字符串轉換爲表明的整數
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       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

My Solution:

1. 這題不難，用一個HashMap存羅馬數字和具體數字的對應關係，而後遍歷先後兩兩比較，該加加，該減減

   • 時間複雜度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));  //這裏map是本身寫的一個方法，裏面用一個switch，至關於HashMap存對應
            int next = map(s.charAt(i+1));
            num = curr < next ? num - curr : num + curr;
            //當時一直想着用一個temp來存減的值，因此無法用for就用了while&point指針，但其實就是很簡單的比後面小的話在總值裏面減去就能夠，不須要temp
        }    
        num += map(s.charAt(n-1));       
        return num;
    }