leetcode day4

【171】Excel Sheet Column Number

Given a column title as appear in an Excel sheet, return its corresponding column number.

For example:

    A -> 1
    B -> 2
    C -> 3
    ...
    Z -> 26
    AA -> 27
    AB -> 28 

思路: 实际上是做一个26进制的数字转换表,用一个数在for循环内纪录字符的位数，从而乘以基数26

public class Solution {
    public int titleToNumber(String s) {
      if(s==null||s.length()==0){
          return 0;
      }
      
      int base = 0;
      for(int i = 0; i<s.length();i++){
          base = base*26 + s.charAt(i)-'A'+1;//当有两位数的时候，base就是个位数*26
      }
      return base;
    }
}

 

逆问题：Excel Sheet Column Title【168】

Given a positive integer, return its corresponding column title as appear in an Excel sheet.

For example:

    1 -> A
    2 -> B
    3 -> C
    ...
    26 -> Z
    27 -> AA
    28 -> AB

思路：给n，用while循环，取出n的每一位，然后对应变换
首先看取出一个数的各个位用Java来实现：

        while(n!=0){
            
            System.out.println("result is "+ n%10);
            n = n/10;
        }//输入1234，则输出4,3,2,1

所以这个也是，依次取出各个位，然后跟A比较后转换

public class Solution {
    public String convertToTitle(int n) {
        StringBuilder result = new StringBuilder();

        while(n!=0){
             result.insert(0, (char)('A' + (n-1) % 26));//因为是26进制，但是是从1开始计数，所以要减一
             n = (n-1)/ 26;
        }

        return result.toString();
    }
}