Remove Duplicates from Sorted Array

Remove Duplicates from Sorted Array | AlgoChronicle

Step 1

Know these before you start

Tick what you already know

Concept video

Step 2

Try it on paper

5–10 minutes, no code yet

ProblemProblem statementGiven an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same.Read

Problem Statement

Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same.

Return k, the number of unique elements. The first k elements of nums must hold the unique elements in their original order; what is left beyond index k does not matter.

Input / Output

Input:  nums = [1,1,2]
Output: 2, nums = [1,2,_]

Input:  nums = [0,0,1,1,1,2,2,3,3,4]
Output: 5, nums = [0,1,2,3,4,_,_,_,_,_]

Constraints

1 ≤ nums.length ≤ 3 * 10^4
-100 ≤ nums[i] ≤ 100
nums is sorted in non-decreasing order

Understanding the Problem

Array is sorted → duplicates are always adjacent
Must be in-place → no second array for the answer
Only the first k slots are checked

Edge Cases

Single element → k = 1
All same → k = 1
All distinct → k = n

Step 6

Get it accepted

All test cases pass

Step 7

Review it so it sticks

Re-solve from memory after

Solve it yourself before you look

Reading the solution first feels like progress, but it makes the next similar problem — and the interview version — much harder, because you skipped the part where you figure it out. Give it an honest 20–30 minutes. Stuck? Re-read the pattern, watch the concept video, or try the brute force first.

Hidden: approach · solution code · optimal solution

Revising this one?

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