leetcode：Excel Sheet Column Title

题目

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 

我的解法

这个其实就是一个进制的转换，相当于将十进制转换为26进制，可以参考
Leetcode：Number of 1 Bits
我的代码：

public class Solution {
    public String convertToTitle(int n) {
        char[] A = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
        String str = "";
        while (n > 26) {
            int reminder = n % 26;
            if (reminder == 0) {
                str = A[25] + str;
                 n--;
            }
            else {
                str = A[reminder - 1] + str;
            }
            n /= 26;
        }
        if (n == 26)
            str = A[25] + str;
        else
            str = A[n - 1] + str;
        return str;
    }
}

缺点
保存26个单词表，浪费空间，后边因为这个数组下标对应问题，折腾好久。

参考答案：

方法一：

及其精炼！！

public class Solution {
    public String convertToTitle(int n) {
        return n == 0 ? "" : convertToTitle(--n / 26) + (char)('A' + (n % 26));
    }
}

方法二:

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

        while(n>0){
            n--;
            result.insert(0, (char)('A' + n % 26));
            n /= 26;
        }

        return result.toString();
    }
}

当然，上述代码中，StringBuilder可换成String，改为：

String column = "";
column = (char)('A' + n % 26) + column;

但是，StringBuilder效率比String要高，具体参考leetcode上的解释：

using simple String basically copies to a new variable, then add the new character to it. so, every time you add a character into a string, it copies the entire string, then adds the new character.【就是说，每次对String执行“+”操作，都会重新创建一个string，显然浪费内存空间】
StringBuilder doesn’t copy that string into a new variable, so it is more memory-efficient

当然有网友提到，如果不采用“头插法”拼接字符串，可以采用多次反转的办法：

1.Reverse each string you want to insert.
2.Append each string to a StringBuilder.
3.Reverse the entire StringBuilder when you’re done.