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4. Remove Duplicates from Sorted Array

easyAsked at LINE

Dedup a sorted array in place — LINE uses this when discussing how to compact dedup'd message IDs in an offline outbox.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given a sorted array nums, remove duplicates in place so each element appears only once and return the new length. Modify the array in place with O(1) extra memory.

Constraints

  • 1 <= nums.length <= 3*10^4
  • nums is sorted non-decreasing.

Examples

Example 1

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

Example 2

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

Approaches

1. Use a Set then copy back

Build a Set, sort entries, and copy them back into the array.

Time
O(n)
Space
O(n)
const u = [...new Set(nums)];
for (let i=0;i<u.length;i++) nums[i]=u[i];
return u.length;

Tradeoff:

2. Two pointers

Track a write index that only advances when the current value differs from the previous kept value.

Time
O(n)
Space
O(1)
function removeDuplicates(nums) {
  if (nums.length === 0) return 0;
  let write = 1;
  for (let read = 1; read < nums.length; read++) {
    if (nums[read] !== nums[read - 1]) {
      nums[write++] = nums[read];
    }
  }
  return write;
}

Tradeoff:

LINE-specific tips

Tie the in-place trick to mobile memory pressure — LINE clients run on low-end Android in Indonesia and Thailand, so allocator-free passes matter.

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Output

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