We have discussed Conversion from column number to Excel Column name. In this post, reverse is discussed.
Given a column title as appears in an Excel sheet, return its corresponding column number.
column column number A -> 1 B -> 2 C -> 3 ... Z -> 26 AA -> 27 AB -> 28
Input: A Output: 1 A is the first column so the output is 1. Input: AA Output: 27 The columns are in order A, B, ..., Y, Z, AA .. So, there are 26 columns after which AA comes.
Approach: The process is similar to binary to decimal conversion.
For example, to convert AB, the formula is 26 * 1 + 2.
As another example,
To convert CDA, 3*26*26 + 4*26 + 1 = 26(3*26 + 4) + 1 = 26(0*26 + 3*26 + 4) + 1
So it is very much similar to converting binary to decimal keeping the base as 26.
Take the input as string and the traverse the input string from the left to right and calculate the result as follows:
result = 26*result + s[i] - 'A' + 1
The result will be summation of
- Time Complexity: O(n), where n is length of input string.
- Space Complexity: O(1).
As no extra space is required.
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