Leetcode: Excel Sheet Column Number

Related to question Excel Sheet Column Title

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

就是把26进制转换为10进制

 1 public class Solution {
 2     public int titleToNumber(String s) {
 3         int res = 0;
 4         for (int i=0; i<s.length(); i++) {
 5             char c = s.charAt(i);
 6             res += (int)(c - 'A' + 1) * Math.pow(26, s.length()-i-1);
 7         }
 8         return res;
 9     }
10 }

从least significant bit开始

26进制

复杂度

时间 O(N) 空间 O(1)

思路

得到数字，其实就是把26进制的数转换为10进制的数。算法就是基本的进制转换方法，从后往前第n位的值乘上26^(n-1)。这里26进制数是1开始的，即A是1。

 1 public class Solution {
 2     public int titleToNumber(String s) {
 3         int res = 0;
 4         int len = s.length();
 5         int k = 1;
 6         for (int i=len-1; i>=0; i--) {
 7             char c = s.charAt(i);
 8             res += (int)(c-'A'+1)*k;
 9             k *= 26;
10         }
11         return res;
12     }
13 }