LeetCode12题——integer to roman

问题描述
oman 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.
解法思路
使用贪心法，相当于找零钱问题，需要注意的是存在6个特例。
C++代码
class Solution {
public:
string intToRoman(int num) {
string s;
string s1(“IVXLCDM”);//罗马数字
if(num4) return “IV”;
if(num9) return “IX”;

    if(num==40) return "XL";
    if(num==90) return "XC";
   
    if(num==400) return "CD";
    if(num==900) return "CM";//特例情况
    while(num>0)//贪心法
    {
        if(num>=1000)
        {
            s+='M';
            num=num-1000;
        }
      
       
        else if(num>=900&&num<1000)
        {
            s+="CM";
            num=num-900;
        }
        else if(num>=500&&num<900)
        {
            s+='D';
            num=num-500;
        }
       
       
        else if(num>=400&&num<500)
        {
            s+="CD";
            num=num-400;
        }
        else if(num>=100&&num<400)
        {
            s+='C';
            num=num-100;
        }
      
        else if(num>=90&&num<100)
        {
            s+="XC";
            num=num-90;
        }
        
        else if(num>=50&&num<90)
        {
            s+='L';
            num=num-50;
        }
       
        else if(num>=40&&num<50)
        {
            s+="XL";
            num=num-40;
        }
        else if(num>=10&&num<40)
        {
            s+='X';
            num=num-10;
        }
       else if(num==9)
       {
           s+="IX";
           num=num-9;
       }
        else if(num>=5&&num<9)
        {
            s+='V';
            num=num-5;
        }
        else if(num==4)
        {
            s+="IV";
            num=num-4;
        }
        else
        {
            s+='I';
            num=num-1;
        }
    }
    return s;
}

};

**结果分析**
Runtime: 44 ms, faster than 97.27% of C++ online submissions for Integer to Roman.
Memory Usage: 31 MB, less than 62.29% of C++ online submissions for Integer to Roman.