[LeetCode]12. Integer to Roman

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,
two is written as II in Roman numeral, just two one's added together.
Twelve is written as, XII, which is simply X + II. The number twenty
seven 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 an integer,
convert it to a roman numeral. Input is guaranteed to be within the
range from 1 to 3999.

Example 1:

Input: 3 Output: "III" Example 2:

Input: 4 Output: "IV" Example 3:

Input: 9 Output: "IX" Example 4:

Input: 58 Output: "LVIII" Explanation: L = 50, V = 5, III = 3. Example
5:

Input: 1994 Output: "MCMXCIV" Explanation: M = 1000, CM = 900, XC = 90
and IV = 4.

這種正向問題，就是找到要求的規律用代碼表達出來，要提升書寫的速度。

public String intToRoman(int num) {
    char[] one=new char[]{'I','X','C','M'};
    char[] five=new char[]{'V','L','D'};
    StringBuilder builder=new StringBuilder();
    while(num>=1000){
        builder.append('M');
        num-=1000;
    }
    if(num==0) return builder.toString();
    for(int i=2;i>=0;i--){
        int t=num/(int)Math.pow(10,i);
        num%=(int)Math.pow(10,i);
        if(t==0) continue;
        else if(t<=3) 
            for(int j=1;j<=t;j++) builder.append(one[i]);
        else if(t==4){
            builder.append(one[i]);
            builder.append(five[i]);
        }else if(t<=8){
            builder.append(five[i]);
            for(int j=6;j<=t;j++) builder.append(one[i]);
        }else{
            builder.append(one[i]);
            builder.append(one[i+1]);
        }
    }
    return builder.toString();
}