LeetCode #989 — EASY

Add to Array-Form of Integer

Build confidence with an intuition-first walkthrough focused on array fundamentals.

The Problem

Problem Statement

The array-form of an integer num is an array representing its digits in left to right order.

For example, for num = 1321, the array form is [1,3,2,1].

Given num, the array-form of an integer, and an integer k, return the array-form of the integer num + k.

Example 1:

Input: num = [1,2,0,0], k = 34
Output: [1,2,3,4]
Explanation: 1200 + 34 = 1234

Example 2:

Input: num = [2,7,4], k = 181
Output: [4,5,5]
Explanation: 274 + 181 = 455

Example 3:

Input: num = [2,1,5], k = 806
Output: [1,0,2,1]
Explanation: 215 + 806 = 1021

Constraints:

1 <= num.length <= 10⁴
0 <= num[i] <= 9
num does not contain any leading zeros except for the zero itself.
1 <= k <= 10⁴

Step 01

Brute Force Baseline

Problem summary: The array-form of an integer num is an array representing its digits in left to right order. For example, for num = 1321, the array form is [1,3,2,1]. Given num, the array-form of an integer, and an integer k, return the array-form of the integer num + k.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Array · Math

Example 1

[1,2,0,0]
34

Example 2

[2,7,4]
181

Example 3

[2,1,5]
806

Core Insight

What unlocks the optimal approach

No official hints in dataset. Start from constraints and look for a monotonic or reusable state.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #989: Add to Array-Form of Integer
class Solution {
    public List<Integer> addToArrayForm(int[] num, int k) {
        List<Integer> ans = new ArrayList<>();
        for (int i = num.length - 1; i >= 0 || k > 0; --i) {
            k += (i >= 0 ? num[i] : 0);
            ans.add(k % 10);
            k /= 10;
        }
        Collections.reverse(ans);
        return ans;
    }
}

// Accepted solution for LeetCode #989: Add to Array-Form of Integer
func addToArrayForm(num []int, k int) (ans []int) {
	for i := len(num) - 1; i >= 0 || k > 0; i-- {
		if i >= 0 {
			k += num[i]
		}
		ans = append(ans, k%10)
		k /= 10
	}
	slices.Reverse(ans)
	return
}

# Accepted solution for LeetCode #989: Add to Array-Form of Integer
class Solution:
    def addToArrayForm(self, num: List[int], k: int) -> List[int]:
        ans = []
        i = len(num) - 1
        while i >= 0 or k:
            k += 0 if i < 0 else num[i]
            k, x = divmod(k, 10)
            ans.append(x)
            i -= 1
        return ans[::-1]

// Accepted solution for LeetCode #989: Add to Array-Form of Integer
impl Solution {
    pub fn add_to_array_form(num: Vec<i32>, k: i32) -> Vec<i32> {
        let mut ans = Vec::new();
        let mut k = k;
        let mut i = num.len() as i32 - 1;

        while i >= 0 || k > 0 {
            if i >= 0 {
                k += num[i as usize];
            }
            ans.push(k % 10);
            k /= 10;
            i -= 1;
        }

        ans.reverse();
        ans
    }
}

// Accepted solution for LeetCode #989: Add to Array-Form of Integer
function addToArrayForm(num: number[], k: number): number[] {
    const ans: number[] = [];
    for (let i = num.length - 1; i >= 0 || k > 0; --i) {
        k += i >= 0 ? num[i] : 0;
        ans.push(k % 10);
        k = Math.floor(k / 10);
    }
    return ans.reverse();
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n)

Space

O(1)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.

Overflow in intermediate arithmetic

Wrong move: Temporary multiplications exceed integer bounds.

Usually fails on: Large inputs wrap around unexpectedly.

Fix: Use wider types, modular arithmetic, or rearranged operations.