171. Excel Sheet Column Number

Time: O(n)

Space: O(1)

Intuition

Base-26 number system where A=1, B=2, ..., Z=26 (no zero digit). Scan left to right: result = result * 26 + (char - "A" + 1).

Algorithm

1result = 0.
2For each character c: result = result * 26 + (c - 'A' + 1).
3Return result.

Example Walkthrough

Input: "ZY"

1.Z: 0*26+26=26. Y: 26*26+25=701.

Output: 701

Common Pitfalls

•This is like converting a base-26 number but with digits 1 - 26 (not 0 - 25).

171.cs

// Approach: Treat the column title as a base-26 number (A = 1…Z = 26),
// accumulating the result left to right with multiplication.
// Time: O(n) Space: O(1)

public class Solution
{
    public int TitleToNumber(string columnTitle)
    {
        return columnTitle.Aggregate(0, (subtotal, c) => subtotal * 26 + (c - 'A' + 1));
    }
}

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