How to Remove Duplicates from a Sorted Array In-Place Using Two Pointers

Remove Duplicates from a Sorted Array

Given a non‑strictly increasing sorted integer array nums, remove duplicate elements in place so that each element appears only once, preserve the relative order, and return the new length k of the array.

Approach

Use two pointers: a slow pointer slow that tracks the position of the last unique element, and a fast pointer fast that scans the array.

Initialize slow = 0 and start fast from index 1.

Whenever nums[fast] != nums[slow], increment slow and copy nums[fast] to nums[slow].

After the loop, the number of unique elements is slow + 1.

Java implementation:

public int removeDuplicates(int[] nums) {
    if (nums == null || nums.length == 0) {
        return 0;
    }
    int slow = 0;
    for (int fast = 1; fast < nums.length; fast++) {
        if (nums[slow] != nums[fast]) {
            slow++;
            nums[slow] = nums[fast];
        }
    }
    return slow + 1;
}

The algorithm runs in O(n) time and O(1) extra space.

Summary : The fast‑slow pointer technique efficiently removes duplicates from a sorted array in a single pass without additional storage.