This problem is the reverse of LeetCode 168.
Instead of converting a number to a column title, here you are given a column title (like used in Excel) and must convert it into the corresponding integer.

Examples:

Input: "A"     → Output: 1
Input: "Z"     → Output: 26
Input: "AA"    → Output: 27
Input: "AB"    → Output: 28
Input: "ZY"    → Output: 701

---

Problem Understanding

Excel column titles behave like a base-26 number system, but instead of digits 0–25, letters represent values 1–26:

A → 1
B → 2
...
Z → 26
AA → 27
AB → 28
...

So the string must be interpreted like a positional numeral:

For "AB":
A contributes 1 * 26¹
B contributes 2 * 26⁰

---

Logic Explained in Simple English

Think of the column title as a number written in a special alphabet-based system.

To convert it into a number:

1. Start with result = 0
2. Read characters from left to right
3. For each character:
   • Convert it to a number between 1 and 26
     Example: ‘C’ → 3
   • Multiply the current result by 26
   • Add the character’s numeric value to result
4. Continue until the end of the string
5. Return result

Why this works:

• Each character acts like a digit position
• Earlier characters are more significant
• Multiplying by 26 shifts previous value left, like base conversion

This is identical to converting a base-26 number to decimal, with letters instead of digits.

---

Step-by-Step Approach

1. Initialize result = 0
2. Loop through each character in the string
3. Convert character to numeric value: value = character - 'A' + 1
4. Update: result = result * 26 + value
5. Return result

---

Java Implementation

class Solution {
    public int titleToNumber(String columnTitle) {
        int result = 0;

        for (char c : columnTitle.toCharArray()) {
            int value = c - 'A' + 1;
            result = result * 26 + value;
        }

        return result;
    }
}

---

Dry Run Example

Example Input: "AB"

Initial:

result = 0

Step 1: Character ‘A’

value = 1
result = 0 * 26 + 1 = 1

Step 2: Character ‘B’

value = 2
result = 1 * 26 + 2 = 28

Final Output:

28

---

Dry Run Example: "ZY"

Initial:

result = 0

Step 1: ‘Z’

value = 26
result = 0 * 26 + 26 = 26

Step 2: ‘Y’

value = 25
result = 26 * 26 + 25 = 676 + 25 = 701

Final Output:

701

---

Time and Space Complexity

Time Complexity

O(n)
(n = length of column title)

Space Complexity

O(1)

---

Common Mistakes and How to Avoid Them

Mistake 1: Treating ‘A’ as 0

In this problem:

A = 1, not 0

Mistake 2: Processing from right to left

Left-to-right is simpler due to multiplication accumulation.

Mistake 3: Using Math.pow

Unnecessary and slower — multiplication is enough.

Mistake 4: Overflow concerns

Not an issue within constraints, but use long if extending.

Mistake 5: Confusing with problem 168

168 converts number to title — this does the reverse.