Integer to English Words

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解题思路：由于最大输入值不可能大于2³¹- 1.即2147483647，第十位：Billion；第七位：Million；第四位：Thousand；第一位：个位。由此可能观察到，将输入值分成四个小组，每三个一组，从低到高依次输入四个小组中。然分别对每个小组中处理，分别百位，十位，个位。

Java实现：

public class Solution {    public String numberToWords(int num) {        if(num==0) return "Zero";        int[] grp=new int[4];        String[] strNum=new String[4];        strNum[0]="";        strNum[1]="";        strNum[2]="";        strNum[3]="";        int i=0;        while(num!=0){        int mod=num%1000;        grp[i++]=mod;                num=num/1000;                }                        for(int j=i;j<grp.length;j++)         grp[j]=0;        for(int k=0;k<grp.length;k++){        int third=grp[k]/100;        if(third!=0){        switch(third){    case 1:strNum[k]+="One ";break;            case 2:strNum[k]+="Two ";break;            case 3:strNum[k]+="Three ";break;            case 4:strNum[k]+="Four ";break;            case 5:strNum[k]+="Five ";break;            case 6:strNum[k]+="Six ";break;            case 7:strNum[k]+="Seven ";break;            case 8:strNum[k]+="Eight ";break;            case 9:strNum[k]+="Nine ";break;    }        strNum[k]+="Hundred ";                }        grp[k]=grp[k]%100;        int second=grp[k]/10;        if(second!=0){        switch(second){                case 1:                    switch(grp[k]){                        case 10:strNum[k]+="Ten ";break;                        case 11:strNum[k]+="Eleven ";break;                        case 12:strNum[k]+="Twelve ";break;                        case 13:strNum[k]+="Thirteen ";break;                        case 14:strNum[k]+="Fourteen ";break;                        case 15:strNum[k]+="Fifteen ";break;                        case 16:strNum[k]+="Sixteen ";break;                        case 17:strNum[k]+="Seventeen ";break;                        case 18:strNum[k]+="Eighteen ";break;                        case 19:strNum[k]+="Nineteen ";break;                    }                                        break;                case 2:strNum[k]+="Twenty ";break;                case 3:strNum[k]+="Thirty ";break;                case 4:strNum[k]+="Forty ";break;                case 5:strNum[k]+="Fifty ";break;                case 6:strNum[k]+="Sixty ";break;                case 7:strNum[k]+="Seventy ";break;                case 8:strNum[k]+="Eighty ";break;                case 9:strNum[k]+="Ninety ";break;                        }        }        if(second!=1){                grp[k]=grp[k]%10;        int first=grp[k];                if(first!=0){        switch(first){    case 1:strNum[k]+="One ";break;            case 2:strNum[k]+="Two ";break;            case 3:strNum[k]+="Three ";break;            case 4:strNum[k]+="Four ";break;            case 5:strNum[k]+="Five ";break;            case 6:strNum[k]+="Six ";break;            case 7:strNum[k]+="Seven ";break;            case 8:strNum[k]+="Eight ";break;            case 9:strNum[k]+="Nine ";break;    }                        }        }        if(strNum[k].length()>0){        switch(k){                case 1:strNum[k]+="Thousand ";break;        case 2:strNum[k]+="Million ";break;        case 3:strNum[k]+="Billion ";break;        }           }                    }        String res=strNum[3]+strNum[2]+strNum[1]+strNum[0];        return res.substring(0,res.length()-1);     }}

题目地址：https://leetcode.com/problems/integer-to-english-words/

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