Check If Digits Are Equal in String After Operations I

Check If Digits Are Equal in String After Operations I

What is this problem about?

The "Check If Digits Are Equal in String After Operations I interview question" is a simulation problem based on digit reduction. You are given a string of digits. In each step, you create a new string where the $i^{th}$ digit is (s[i] + s[i+1]) % 10. You repeat this process until only two digits remain. You need to return true if those two final digits are equal.

Why is this asked in interviews?

Companies like Microsoft and Meta use the "Check If Digits Are Equal coding problem" to test a candidate's ability to implement an iterative simulation. It evaluates basic loop handling, string manipulation, and modular arithmetic. It’s a test of whether you can correctly follow a step-by-step procedure and handle decreasing string lengths.

Algorithmic pattern used

This problem follows the Linear Simulation pattern.

1. Looping: Use a while loop that continues as long as the string length is greater than 2.
2. String Building: In each iteration, create a temporary string or list.
3. Digit Math: Iterate from 0 to len - 2, calculate the sum of adjacent digits modulo 10, and append to the new string.
4. Update: Replace the old string with the new one.
5. Compare: After the loop, compare the two remaining characters.

Example explanation

String: "1234"

1. Step 1:
   • (1+2)%10 = 3
   • (2+3)%10 = 5
   • (3+4)%10 = 7
   • New string: "357"
2. Step 2:
   • (3+5)%10 = 8
   • (5+7)%10 = 2
   • New string: "82"
3. Comparison: $8 eq 2$. Result: False.

Common mistakes candidates make

• Off-by-one: Stopping the inner loop too late or too early, leading to index out of bounds.
• Modulo: Forgetting to apply % 10 at each step.
• Efficiency: Using string concatenation s += new_char in languages like Java or Python, which is $O(N^2)$ inside the loop. Using a character array or list is much better.

Interview preparation tip

For simulation problems with small constraints, focus on correctness over extreme optimization. However, always use a StringBuilder or list join to avoid the $O(N^2)$ string concatenation pitfall—this is a classic "Simulation interview pattern" point of feedback.