LeetCode: Roman to Integer 解题报告

Roman to Integer
Given a roman numeral, convert it to an integer.

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

SOLUTION 1： 

思路：

从后往前遍历罗马数字，如果某个数比前一个数小，则把该数在结果中减掉；
反之，则在结果中加上当前这个数；

package Algorithms.string;

public class RomanToInt {
    public int romanToInt(String s) {
        if (s == null) {
            return 0;
        }
        
        int len = s.length();
        int sum = 0;
        int pre = 0;
        
        for (int i = len - 1; i >= 0; i--) {
            int cur = romanTable(s.charAt(i));
            
            if (i == len - 1) {
                // 如果是在尾部，直接加上当前值
                sum += cur;
            } else {
                // 判定当前值是不是比前一个值要小，如果小，则需要减去它
                if (cur < pre) {
                    sum -= cur;
                } else {
                    sum += cur;
                }
            }
            pre = cur;
        }
        
        return sum;
    }
    
    public int romanTable(char c) {
        int num = 0;
        switch(c) {
            case 'I':
                num = 1;
                break;
            case 'V':
                num = 5;
                break;
            case 'X':
                num = 10;
                break;
            case 'L':
                num = 50;
                break;
            case 'C':
                num = 100;
                break;
            case 'D':
                num = 500;
                break;
            case 'M':
                num = 1000;
                break;
            default:
                num = 0;
                break;
        }
        
        return num;
    }
}

SOLUTION 2:

除了用函数转换，也可以用map来转换。

public int romanToInt(String s) {
        if (s == null) {
            return 0;
        }
        
        // bug 1: forget new.
        HashMap<Character, Integer> map = new HashMap<Character, Integer>();
        map.put('I', 1);
        map.put('V', 5);
        map.put('X', 10);
        map.put('L', 50);
        map.put('C', 100);
        map.put('D', 500);
        map.put('M', 1000);
        
        int len = s.length();
        int num = 0;
        for (int i = len - 1; i >= 0; i--) {
            int cur = map.get(s.charAt(i));
            if (i < len - 1 && cur < map.get(s.charAt(i + 1))) {
                num -= cur;
            } else {
                num += cur;
            }
        }
        
        return num;
    }

GITHUB 代码：

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/string/RomanToInt.java