Remove Duplicates from Sorted Array Algorithm

You have a sorted array of integers. Remove duplicates in-place so each value appears only once. Return the count of unique elements.

The catch: you can't allocate a new array. Modify the original. Only the first k elements matter — whatever's left beyond that is irrelevant.

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

The intuition

Since the array is already sorted, duplicates sit next to each other. Use two pointers: one slow (writeIndex) to mark where the next unique element goes, and one fast (i) to scan ahead.

When nums[i] differs from nums[writeIndex], we found a new unique value. Copy it forward and advance the write pointer.

var removeDuplicates = function(nums) {
    if (nums.length === 0) return 0;

    let writeIndex = 1;

    for (let i = 1; i < nums.length; i++) {
        if (nums[i] !== nums[writeIndex - 1]) {
            nums[writeIndex] = nums[i];
            writeIndex++;
        }
    }

    return writeIndex;
};

Complexity

• Time: O(n) — one pass.
• Space: O(1) — in-place.

The trade-off

This only works because the input is sorted. Unsorted arrays need a different strategy (usually a Set, which costs O(n) space). Always check your preconditions before picking an approach.