#140 Easy Time O(n) Space O(1) Animated

Plus One

Increment a large integer represented as an array of digits.

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2 Trace
3 Explain
4 Implement

Pattern Direct simulation with preserved structure

Name the mathematical relationship first, then apply it consistently to every position or digit.

Approach: Array Iteration

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Core Insight

Name the mathematical relationship first, then apply it consistently to every position or digit.

Common Pitfall

Write down index and boundary conditions before mutating an in-place structure.

Description

Increment a large integer represented as an array of digits.

Constraints

Time Complexity: O(n)
Space Complexity: O(1)

Solution — Array Iteration

TimeO(n) SpaceO(1)

Implementation Plan

Translate the invariant into these small, testable moves.

Identify the formula or transformation.
Set boundary conditions.
Process each required position.
Return the transformed result.

def plusOne(digits):
    digits = digits[::-1]
    one, i = 1, 0
    
    while one:
        if i < len(digits):
            if digits[i] == 9:
                digits[i] = 0
            else:
                digits[i] += 1
                one = 0
        else:
            digits.append(1)
            one = 0
        i += 1
        
    return digits[::-1]

Happy Number

Pow(x, n)

Plus One

Steps

Description

Constraints

Tags

Solution — Array Iteration