Remove Duplicates from a Sorted Array

Problem

Remove duplicates from a sorted array in place and return the count of unique elements. The first part of the array should contain the unique elements.

Input:  nums = [1, 1, 2, 2, 3]
Output: k = 3, nums = [1, 2, 3, ...]

Python Solution

def remove_duplicates(nums):
    if not nums:
        return 0
    slow = 0
    for fast in range(1, len(nums)):
        if nums[fast] != nums[slow]:
            slow += 1
            nums[slow] = nums[fast]
    return slow + 1

C++ Solution

int removeDuplicates(vector<int>& nums) {
    if (nums.empty()) return 0;
    int slow = 0;
    for (int fast = 1; fast < nums.size(); fast++) {
        if (nums[fast] != nums[slow]) {
            slow++;
            nums[slow] = nums[fast];
        }
    }
    return slow + 1;
}

Java Solution

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

Complexity

Time: O(n) — the fast pointer visits every element once. Space: O(1) — duplicates are overwritten in place using the slow pointer.