168. 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 

链接： http://leetcode.com/problems/excel-sheet-column-title/

题解：

转换数字和excel column title。数学题，写几个例子找到规律就可以了，比如53, 52, 26, 27等等。规律是，当n > 0时， 假如n % 26 == 0，则n为26的倍数，要转换为'Z'插入队首，之后n = n / 26 - 1， 假如n % 26 != 0， 则使用一般方法计算字符，int c = 'A' + n % 26 - 1， 之后 n /= 26。

Time Complexity - O(N)， Space Complexity - O(1)。

public class Solution {
    public String convertToTitle(int n) {
        if(n <= 0)
            return "";
        StringBuilder result = new StringBuilder();
        
        while(n > 0) {
            if(n % 26 == 0) {           // n is multiple of 26,  example - 52 - 'AZ'
                result.insert(0, 'Z');
                n = n / 26 - 1;
            } else {
                int c = 'A' + n % 26 - 1;   // example - 27 - 'AA'
                result.insert(0, (char)c);
                n /= 26;
            }
        }
        
        return result.toString();
    }
}

二刷:

二刷好好参考了解答。 使用了一个很巧妙的小技术，把n映射到0-25这26个数字里，我们可以先n--， 然后再进行后面的转换 -  n % 26 + 'A'，  最后n /= 26.

这个小技巧和Permutation Sequence很像，要好好学习。

Java：

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

三刷:

二刷理解得不深刻。

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

一刷的方法，用例子出发  - 'AZ' = 52,  - 'AA' = 27

public class Solution {
    public String convertToTitle(int n) {
        if (n <= 0) return "";
        StringBuilder sb = new StringBuilder();
        while (n > 0) {
            if (n % 26 == 0) {
                sb.insert(0, 'Z');          // 52 = "AZ"
                n = n / 26 - 1;
            } else {
                sb.insert(0, (char)(n % 26 - 1 + 'A'));  // 27 = "AA"
                n = n / 26;
            }
        }
        return sb.toString();
    }
}

Reference：

https://leetcode.com/discuss/19047/my-1-lines-code-in-java-c-and-python

https://leetcode.com/discuss/19044/share-simple-solution-just-little-trick-handle-corner-case

https://leetcode.com/discuss/34526/share-my-java-solusion

https://leetcode.com/discuss/25667/short-solution-without-using-string-buffer-string-builder

https://leetcode.com/discuss/19135/solution-with-explanation

https://leetcode.com/discuss/82531/my-easy-to-understand-java-solution