➕ 66. Plus One

Difficulty: 🟢 Easy
Topics: Arrays, Math

📘 Problem Statement

You are given a large integer represented as an array of digits called digits.

• Each digits[i] represents a single digit.
• The digits are ordered from most significant to least significant (left → right).
• The number does not contain any leading zeros.

🎯 Goal

Increment the number by one and return the resulting array of digits.

🧠 Examples

✅ Example 1

Input:  [1, 2, 3]
Output: [1, 2, 4]

Explanation:
The array represents the number 123.
After incrementing: 123 + 1 = 124

✅ Example 2

Input:  [4, 3, 2, 1]
Output: [4, 3, 2, 2]

Explanation:
The array represents the number 4321.
After incrementing: 4321 + 1 = 4322

✅ Example 3

Input:  [9]
Output: [1, 0]

Explanation:
The array represents the number 9.
After incrementing: 9 + 1 = 10

📌 Constraints

• 1 ≤ digits.length ≤ 100
• 0 ≤ digits[i] ≤ 9
• No leading zeros in the input array

✨ Tip:
Think about how carry works when the last digit is 9!

public class Main {

public static int[] plusOne(int[] digits) {
    int n = digits.length;

    // Traverse from last digit
    for (int i = n - 1; i >= 0; i--) {
        if (digits[i] < 9) {
            digits[i]++;      // simple increment
            return digits;    // done
        }
        digits[i] = 0;        // carry forward
    }

    // If all digits were 9 (e.g., 9, 99, 999)
    int[] result = new int[n + 1];
    result[0] = 1; // leading 1
    return result;
}

public static void main(String[] args) {
    int[] digits = {9, 9, 9};

    int[] result = plusOne(digits);

    // Print result
    for (int digit : result) {
        System.out.print(digit + " ");
    }
}

}