Plus One

LeetCode 66 | Difficulty: Easy

Easy

Problem Description

You are given a large integer represented as an integer array digits, where each digits[i] is the i^th digit of the integer. The digits are ordered from most significant to least significant in left-to-right order. The large integer does not contain any leading 0's.

Increment the large integer by one and return the resulting array of digits.

Example 1:

Input: digits = [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.
Incrementing by one gives 123 + 1 = 124.
Thus, the result should be [1,2,4].

Example 2:

Input: digits = [4,3,2,1]
Output: [4,3,2,2]
Explanation: The array represents the integer 4321.
Incrementing by one gives 4321 + 1 = 4322.
Thus, the result should be [4,3,2,2].

Example 3:

Input: digits = [9]
Output: [1,0]
Explanation: The array represents the integer 9.
Incrementing by one gives 9 + 1 = 10.
Thus, the result should be [1,0].

Constraints:

- `1 <= digits.length <= 100`

- `0 <= digits[i] <= 9`

- `digits` does not contain any leading `0`'s.

Topics: Array, Math

---

Approach

Mathematical

Look for mathematical patterns or formulas. Consider: modular arithmetic, GCD/LCM, prime factorization, combinatorics, or geometric properties.

When to use

Problems with clear mathematical structure, counting, number properties.

---

Solutions

Solution 1: C# (Best: 588 ms)

Metric	Value
Runtime	588 ms
Memory	N/A
Date	2018-03-08

Solution

public class Solution {
    public int[] PlusOne(int[] digits) {
        int carry = 1;
    List<int> result = new List<int>();
    for (int i = digits.Length-1; i >= 0; i--)
    {
        if(digits[i] == 9 && carry==1)
        {
            carry=1;
            result.Add(0);
        }
        else
        {
            result.Add(digits[i]+carry);
            carry=0;
        }
    }
    if(carry==1) result.Add(carry);
    result.Reverse();
    return result.ToArray();
    }
}

---

Complexity Analysis

Approach	Time	Space
Solution	$O(n)$	$O(1) to O(n)$

---

Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.