What is the brute force approach and complexity for Remove Duplicates from Sorted Array?

Insert all values into a set, then copy the set back with time complexity O(N log N) and space complexity O(N).

What is the optimal approach and complexity for Remove Duplicates from Sorted Array?

Keep nums[read] only if it differs from nums[write − 1] with optimal time complexity O(N) and space complexity O(1).

How do you identify the pattern for Remove Duplicates from Sorted Array?

Look for key signals in the prompt: "sorted in non-decreasing order" indicates Copies of a value are next to each other → one comparison catches a repeat.; "remove … in place, keep the order" indicates Build the answer at the front of `nums` with a `write` pointer.; "return k, first k elements are checked" indicates The answer is the prefix `nums[0 … write−1]`..

Remove Duplicates from Sorted Array

Easy·LeetCode #26 Pro OnlyBasic Reinforcement

LeetCode

Problem Statement

You are given an integer array nums sorted in non-decreasing order (it may contain repeated values).

Remove the repeats in place so that each value appears only once, and keep the values in their original order. Return k, the number of unique values.

After your function runs, the first k slots of nums must hold the unique values in increasing order. Whatever is left after slot k − 1 is ignored.

Input

An integer array nums, sorted in non-decreasing order.

Output

An integer k. The judge also checks that nums[0 … k−1] is exactly the unique values, in order.

Example 1

Input: nums = [1, 1, 2]

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

The unique values are 1 and 2. They go in the first 2 slots.

Example 2

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

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

Each of 0, 1, 2, 3, 4 is kept once, in order.

Constraints

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

Pattern Recognition Clues

Recognising these key signals in the problem description immediately identifies the pattern:

“sorted in non-decreasing order”

→ Copies of a value are next to each other → one comparison catches a repeat.

“remove … in place, keep the order”

→ Build the answer at the front of `nums` with a `write` pointer.

“return k, first k elements are checked”

→ The answer is the prefix `nums[0 … write−1]`.

Approach Complexity Ladder

Brute Force — put everything in a setO(N log N)O(N)

Insert all values into a set, then copy the set back

Optimal — read / write, compare with the last kept valueO(N)O(1)

Keep nums[read] only if it differs from nums[write − 1]

Brute Force Intuition

A set keeps only one copy of each value and stores them sorted. Insert every element, then copy the set back into the front of nums and return its size.

Recognition rule for this sub-pattern:

One pointer reads every element, the other marks where the next "kept" element should be written.

Remember this · Two Pointers

Trigger

Sorted array, pairs/triplets with a target, comparing from both ends, compacting an array in place.

Think

If I compare the elements at two positions, can I safely discard one side?

Key question

Which pointer moves, and why can the skipped elements never be the answer?

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