Problem statement

Given a string columnTitle that represents the column title as appears in an Excel sheet, return its corresponding column number.

For example:

A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28 
...

Example 1

Input: columnTitle = "A"
Output: 1

Example 2

Input: columnTitle = "AB"
Output: 28

Example 3

Input: columnTitle = "ZY"
Output: 701

Constraints

• 1 <= columnTitle.length <= 7.

• columnTitle consists only of uppercase English letters.

• columnTitle is in the range ["A", "FXSHRXW"].

Solution: Finding The Pattern

Let us write down some other columnTitle strings and its value.

"A"   =  1
"Z"   =  26
"AA"  =  27
"AZ"  =  52
"ZZ"  =  702
"AAA" =  703

Then try to find the pattern

"A"   =  1    =  1
"Z"   =  26   =  26
"AA"  =  27   =  26 + 1
"AZ"  =  52   =  26 + 26
"ZZ"  =  702  =  26*26 + 26
"AAA" =  703  =  26*26 + 26 + 1

If you map 'A' = 1, ..., 'Z' = 26, the values can be rewritten as

"A"   =  1    =  'A'
"Z"   =  26   =  'Z'
"AA"  =  27   =  26*'A' + 'A'
"AZ"  =  52   =  26*'A' + 'Z'
"ZZ"  =  702  =  26*'Z' + 'Z'
"AAA" =  703  =  26*26*'A' + 26*'A' + 'A'

In general the formula for a string columnTitle = abcd is

abcd = 26^3*a + 26^2*b + 26*c + d,

where a, b, c, d are some uppercase English letters A, ..., Z.

Longer columnTitles will have bigger leading exponents of 26.

Code

#include <iostream>
using namespace std;
int titleToNumber(string columnTitle) {
    int column = 0; 
    for (char c : columnTitle) {
        // The ASCII value of 'A' is 65.
        column = 26*column + (c - 64); 
    }
    return column;
}
int main() {
    cout << titleToNumber("A") << endl;
    cout << titleToNumber("AB") << endl;
    cout << titleToNumber("ZY") << endl;
}

Output:
1
28
701

Complexity

• Runtime: O(N), where N = columnTitle.length.

• Extra space: O(1).

Implementation notes

1. There are many ways to compute the series

    26^3*a + 26^2*b + 26*c + d.
   

   If you write it as

    26*(26*(26*(0 + a) + b) + c) + d,
   

   you get the loop in the code above.

2. To map 'A' = 1, ..., 'Z' = 26, you can use their ASCII values ('A' = 65, ..., 'Z' = 90) minus 64.

3. The parentheses around (c - 64) is needed. Otherwise the value of columnTitle = "FXSHRXW" makes 26*column + c exceed the limit of int before it substracts 64.

References

• https://leetcode.com/problems/excel-sheet-column-number/

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