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

public class Solution {
    public int titleToNumber(String s) {
        int res = 0;
        for (int i=0; i<s.length(); i++) {
            char c = s.charAt(i);
            res += (int)(c - 'A' + 1) * Math.pow(26, s.length()-i-1);
        }
        return res;
    }
}

 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。

public class Solution {
    public int titleToNumber(String s) {
        int res = 0;
        int len = s.length();
        int k = 1;
        for (int i=len-1; i>=0; i--) {
            char c = s.charAt(i);
            res += (int)(c-'A'+1)*k;
            k *= 26;
        }
        return res;
    }
}