题目：

题解：

char* singles[] = {"", "One ","Two ","Three ","Four ","Five ","Six ","Seven ","Eight ","Nine "};
char* teens[] = {"Ten ","Eleven ","Twelve ","Thirteen ","Fourteen ","Fifteen ","Sixteen ","Seventeen ","Eighteen ","Nineteen "};
char* tys[] = {"","Ten ","Twenty ","Thirty ","Forty ","Fifty ","Sixty ","Seventy ","Eighty ","Ninety "};

char * toEnglish(char* res, int num){				//转英文函数
    int curNum = num;

    int hundred = curNum / 100;    					//百位以上
    if(hundred != 0){								//是否大于百位
        strcat(res, singles[hundred]);
        strcat(res, "Hundred ");
    }
    curNum %= 100;									//百位以下
    int ty = curNum / 10;							//十位~百位
    if(ty >= 2){									//是否大于20
        strcat(res, tys[ty]);
        curNum %= 10;
    }
    if(curNum >= 10){								//10~20
        strcat(res, teens[curNum-10]);
    }else if(curNum > 0 && curNum < 10){			//个位
        strcat(res, singles[curNum]);
    }

    return res;
}

char * numberToWords(int num){
    if(num == 0)
        return "Zero";
    
    char* res = (char*)malloc(sizeof(char) * 200);	//储存结果
    res[0] = '\0';
    int curNum = num;

    int billion = curNum / 1000000000;				//十亿以上
    if(billion != 0){								//是否大于十亿
        toEnglish(res, billion);					
        strcat(res, "Billion ");
    }
    curNum %= 1000000000;							//十亿以下
    int million = curNum / 1000000;					//百万~十亿
    if(million != 0){								//是否大于百万
        toEnglish(res, million);
        strcat(res, "Million ");
    }
    curNum %= 1000000;								//百万以下
    int thousand = curNum / 1000;					//千~百万
    if(thousand != 0){								//是否大于千位
        toEnglish(res, thousand);
        strcat(res, "Thousand ");
    }
    curNum %= 1000;									//千位以下
    toEnglish(res, curNum);
    
    int len = strlen(res);
    res[len-1] = '\0';								//字符串结束

    return res;
}