Integer to something 类型题总结

这种类型题的思路就是将可能的情况放到Array中，由小到大或者由大到小排列，将需要得到的结果依次减Array中的index得出结果。之前做过的Broadway Technology的OA也是同样类型的题。

12. Integer to Roman：

class Solution {
    public String intToRoman(int num) {
        int[] romanNumber = new int[]{1, 4, 5, 9, 10, 40, 50, 90, 100, 400, 500, 900, 1000};
        String[] strs = new String[]{"I", "IV", "V", "IX", "X", "XL", "L", "XC", "C", "CD", "D", "CM", "M"};
        
        String ans = "";
        int index = romanNumber.length-1;
        while(num>0){
            if(romanNumber[index]>num){
                index--;
                continue;
            }
            else {
                num = num - romanNumber[index];
                ans = ans + strs[index];
            }
        }
        return ans;
        
    }
}

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273. Integer to English Words

这道题的思想其实与罗马数字也类似，但不同点在于罗马数字是不停地减，这种需要不停地取余，相当于从高位到低位依次处理。
分析这道题的时候，关键是发现：数字在西方是以1000为进制的，所以最基本的处理是针对1000来操作，1000以上的数只需要在1000的基础上稍加处理。所以如果是处理中文的话，进制是一万。

class Solution {
    public String numberToWords(int num) {
        if(num==0)
            return "Zero";
        String ans = "";
        while(num>0){
            if(num>=1000000000){
                int billionNum = num/1000000000;
                ans = ans + computeNumLessThousand(billionNum) + "Billion ";
                num = num % 1000000000;
            } 
            else if(num>=1000000){
                int millionNum = num/1000000;
                ans = ans + computeNumLessThousand(millionNum) + "Million ";
                num = num % 1000000;                
            }
            else if(num>=1000){
                int thousandNum = num/1000;
                ans = ans + computeNumLessThousand(thousandNum) + "Thousand ";
                num = num % 1000;                  
            }
            else{
                ans = ans + computeNumLessThousand(num);
                break;
            }
        }
        ans = ans.substring(0, ans.length()-1);
        return ans;
        
    }
    
    private String computeNumLessThousand(int num){
        String[] digits = new String[]{"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        String[] tens = new String[]{"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        String ans = "";
        while(num>0){
            if(num>=100){
                int hundredNum = num/100;
                ans = ans + computeNumLessThousand(hundredNum) + "Hundred ";
                num = num %100;
            }
            else if(num>=20){
                int tensNum = num/10;
                ans = ans + tens[tensNum] + " ";
                num = num %10;
            }
            else {
                ans = ans + digits[num] + " ";
                break;
            }
        }
        return ans;
    }
}