Roman Number & Integer

Problem I: Roman to Integer

Given a roman numeral, convert it to an integer.

Input is guaranteed to be within the range from 1 to 3999.

解决思路

1. 将字符和整数存入map中；

2. 从后往前扫描，如果当前字符代表的值大于后一个字符的值，则用当前的值减去之前累计的值。e.g. "IV" -> 5 - 1 = 4

程序

public class Solution {
    public int romanToInt(String s) {
        if (s == null || s.length() == 0) {
		    return 0;
	    }
	    
    	HashMap<Character, Integer> hm = new HashMap<Character, Integer>();
    	hm.put('I', 1);
    	hm.put('V', 5);
    	hm.put('X', 10);
    	hm.put('L', 50);
    	hm.put('C', 100);
    	hm.put('D', 500);
    	hm.put('M', 1000);
    	
    	int len = s.length();
    	int sum = hm.get(s.charAt(len - 1));
    	
    	for (int i = len - 2; i >= 0; i--) {
    	    char cur = s.charAt(i);
    	    char aft = s.charAt(i + 1);
    	    if (hm.get(cur) < hm.get(aft)) {
    	        sum -= hm.get(cur);
    	    } else {
    	        sum += hm.get(cur);
    	    }
    	}
    	
    	return sum;
    }
}

Problem II: Integer to Roman

Given an integer, convert it to a roman numeral.

Input is guaranteed to be within the range from 1 to 3999.

解决思路

1. 存一个数字到字符的映射表，数字从大到小，包含{1000, 900, 500, 400, 100, 90,
            50, 40, 10, 9, 5, 4, 1}；

2. 贪心原则，得到对应的字符串。

程序

public class Solution {
    public String intToRoman(int num) {
        String res = "";
        if (num <= 0) {
			return res;
		}
		
		int[] nums = {1000, 900, 500, 400, 100, 90, 
			50, 40, 10, 9, 5, 4, 1};
		String[] symbols = {"M", "CM", "D", "CD", "C", "XC",
			"L", "XL", "X", "IX", "V", "IV", "I"};
		
		int i = 0;
		
		while (num > 0) {
		    int j = num / nums[i];
		    num -= j * nums[i];
		    for ( ; j > 0; j--) {
		        res += symbols[i];
		    }
		    ++i;
		}
		
		return res;
    }
}