LeetCode Challenge Day 38 — 3461. Check If Digits Are Equal in String After Operations I

Hey folks

This is Day 38 of my LeetCode streak 🚀.
Today’s problem is 3461. Check If Digits Are Equal in String After Operations I — an easy simulation problem where we repeatedly reduce a string of digits until only two remain, then check if those two digits are equal.

📌 Problem Statement

You are given a string s consisting of digits. Perform the following operation until s has exactly two digits:

• For each pair of consecutive digits in s, calculate a new digit as (s[i] + s[i+1]) % 10.
• Replace s with the sequence of these new digits.

Return true if the final two digits are equal, otherwise return false.

💡 Intuition

The key observation is that we don’t need to optimize anything — the input size is at most 100, so we can directly simulate the process.

At each step, the length of the string decreases by 1, and after at most n-2 steps we’ll reach exactly two digits.

🔑 Approach

1. Convert the input string into an array of digits.
2. While the array length is greater than 2:
   • Create a new array.
   • For each consecutive pair, compute (arr[i] + arr[i+1]) % 10.
   • Replace the old array with this new one.
3. Once only 2 digits remain, check if they are equal.

⏱️ Complexity Analysis

• Time complexity:
  Each step reduces the array length by 1, and each step costs up to O(n).
  Total complexity: O(n²), which is fine for n ≤ 100.

• Space complexity:
  We only store arrays proportional to the size of the input.
  So, O(n).

🧑‍💻 Code (JavaScript)

/**
 * @param {string} s
 * @return {boolean}
 */
var hasSameDigits = function(s) {
    // Convert string to array of digits
    let arr = s.split("").map(Number);

    // Keep reducing until only 2 digits left
    while (arr.length > 2) {
        let next = [];
        for (let i = 0; i < arr.length - 1; i++) {
            next.push((arr[i] + arr[i + 1]) % 10);
        }
        arr = next;
    }

    // Check if last two digits are equal
    return arr[0] === arr[1];
};

🧪 Example Walkthrough

Input:
s = "3902"

Steps:

• First reduction: "3902" → "292"

• Second reduction: "292" → "11"

Since the last two digits are the same, the result is true ✅.

🎥 Reflections

This problem is a great reminder that sometimes brute-force simulation is the simplest and most effective strategy when input sizes are small.
It also reinforces the importance of checking constraints before over-optimizing.

That’s it for Day 38 of my LeetCode journey!
Onwards to the next challenge 🔥

Happy Coding 👨‍💻