LeetCode题解-Excel表列序号-Java

Excel表列序号题解，这个和罗马数字转换比较类似，都是比较实用的题。
下面是题解，可以参考

package easy;

import java.util.HashMap;
import java.util.Map;

public class TitleToNumber {
   
    public static void main(String[] args) {
   
        System.out.println(new TitleToNumber().titleToNumber("Z"));
    }
    public int titleToNumber1(String columnTitle) {
   
        Map<Character,Integer> char2num=new HashMap<>();
        for (int i = 0,j=1; i < 26; i++,j++) {
   
            char2num.put((char)('A'+i),j);
        }
        char[] chars = columnTitle.toCharArray();
        int result=0;
        int power=1;
        for (int i = chars.length - 1; i >= 0; i--) {
   
            result+=char2num.get(chars[i])*power;
            power*=26;
        }
        return result;
    }
    public int titleToNumber2(String columnTitle) {
   
        char[] chars = columnTitle.toCharArray();
        int result=0;
        int power=1;
        for (int i = chars.length - 1; i >= 0; i--) {
   
            result+=(chars[i]-'A'+1)*power;
            power*=26;
        }
        return result;
    }

    public int titleToNumber(String columnTitle) {
   
        int result=0;
        int power=1;
        char m='A'-1;
        for (int i = columnTitle.length() - 1; i >= 0; i--) {
   
            result+=(columnTitle.charAt(i)-m)*power;
            power*=26;
        }
        return result;
    }
}