Leetcode: Excel Sheet Column Title

Given a positive integer, return its corresponding column title as appear in an Excel sheet.For example:1 -> A2 -> B3 -> C...26 -> Z27 -> AA28 -> AB

这道题是我微软onsite时遇到的一道题，没做过遇到这道题确实有点难一下子理得很清楚（我当时这道题应该做的不好，从most significant digit做，而且忘了n要-1）。这道题说白了其实就是十进制转换26进制，而且是从1开始的1十进制的转换

本质是进制转换，将n转化为26进制，转化过程如下（括号里的是26进制数）：

1->(1)->A

2->(2)->B

...

26->(10)->Z

27->(11)->AA

28->(12)->AB

.....

52->(20)->AZ

53->(21)->BA

从least significant digit开始，不断地除以26取余数

这是我的方法：因为A是1，而不是0，相当于26进制的数都整体减1，才能对应上从0开始的十进制数。

 1 public class Solution {
 2     public String convertToTitle(int n) {
 3         if (n <= 0) return "";
 4         StringBuffer res = new StringBuffer();
 5         while (n > 0) {
 6             res.insert(0, (char)('A' + (n-1)%26));
 7             n = (n-1) / 26;
 8         }
 9         return res.toString();
10     }
11 }

这道题首先因为26个数字为一组，26次变一次，所以肯定是26进制，如果是1-26, 那么26号数字没法跟前面保持一致，比如都是一位25/26=0 而26/26=1. 所以应该回归0-based，1-26各数减一变成0-25，对应A到Z。但新的问题又出现了：AA本来是27，减了一之后是26, 26%26==0，最后一位是A没错，但是前一位26/26 == 1，又对应A，刚才0才对应A来着。所以，每一循环都要减一，以确保是0-based

另一种做法：

 1 public String convertToTitle(int n) {
 2     if(n <= 0){
 3         throw new IllegalArgumentException("Input is not valid!");
 4     }
 5
 6     StringBuilder sb = new StringBuilder();
 7
 8     while(n > 0){
 9         n--;
10         char ch = (char) (n % 26 + 'A');
11         n /= 26;
12         sb.append(ch);
13     }
14
15     sb.reverse();
16     return sb.toString();
17 }

转载于:https://www.cnblogs.com/EdwardLiu/p/4179867.html

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