https://leetcode.com/problems/excel-sheet-column-number/\#/description

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

思路:

formula:

假设 abcd 为column number，a,b,c,d都是变量，范围是 A - Z (1 - 26)，对应的number n 为：

n = a * 26^3 + b * 26^2 + c * 26^1 + d * 26^0

Q1: 如何求出对应的d呢？

假设 d1 = d - 1 ， d1的范围[0, 25] 依次对应相应的A - Z

==> n = a * 26^3 + b * 26^2 + c * 26^1 + (d1 + 1) * 26^0

==> n - 1= a * 26^3 + b * 26^2 + c * 26^1 + d1

==> d1 = (n - 1) % 26

(n - 1) / 26 = a * 26^2 + b * 26^1 + c (jump to Q1 again)

Code相应为：

public class Solution {
    public String convertToTitle(int n) {
        StringBuilder sb = new StringBuilder();
        while(n > 0) {
            char c = (char)('A' + (n - 1) % 26);
            n = (n - 1) / 26;
            sb.insert(0, c);
        }
        return sb.toString();
    }
}