LeetCode Challenge Day 108 — 66. Plus One

Hey folks 👋

This is Day 108 of my LeetCode streak 🚀
Today's problem is 66. Plus One — an easy problem on paper, but a great test of clean thinking and edge-case handling.

📌 Problem Statement

You are given:

• An array digits representing a non-negative integer
• Each element is a single digit (0–9)
• Digits are ordered from most significant to least significant
• No leading zeroes

Goal:
Increment the number by one and return the resulting digits array.

💡 Intuition

This problem is just elementary addition, applied to an array.

Key observations:

• Addition starts from the last digit
• If a digit is less than 9, increment it and stop
• If a digit is 9, it becomes 0 and generates a carry
• If all digits are 9, the carry propagates through the entire array and creates a new leading 1

So the entire problem boils down to handling carry correctly.

🔑 Approach

1. Traverse the array from right to left
2. For each digit:
   • If it’s less than 9, increment it and return immediately
   • Otherwise, set it to 0 and continue
3. If the loop completes, all digits were 9
   • Insert 1 at the beginning of the array
4. Return the updated array

This avoids converting the array into a number and works efficiently for large inputs.

⏱️ Complexity Analysis

• Time Complexity:
  O(n) — single pass through the digits

• Space Complexity:
  O(1) — in-place modification (excluding output)

🧑‍💻 Code (JavaScript)

/**
 * @param {number[]} digits
 * @return {number[]}
 */
var plusOne = function(digits) {
    for (let i = digits.length - 1; i >= 0; i--) {
        if (digits[i] < 9) {
            digits[i]++;
            return digits;
        }
        digits[i] = 0;
    }
    
    // If all digits were 9
    digits.unshift(1);
    return digits;
};

🎯 Reflection

This problem reinforces an important lesson:

• Simple problems test discipline, not difficulty

• Carry handling is a recurring pattern in many harder problems

• Clean early returns keep the code readable

That wraps up Day 108 of my LeetCode challenge 🔥
On to Day 109 — consistency beats intensity 🚀

Happy Coding 👨‍💻