26. Remove Duplicates from Sorted Array

1. Description

Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. The relative order of the elements should be kept the same.
Consider the number of unique elements in nums to be k​​​​​​​​​​​​​​. After removing duplicates, return the number of unique elements k.
The first k elements of nums should contain the unique numbers in sorted order. The remaining elements beyond index k - 1 can be ignored.

2. Example

Example 1

Input: nums = [1,1,2]
Output: 2, nums = [1,2,_]
Explanation: Your function should return k = 2, with the first two elements of nums being 1 and 2 respectively.
It does not matter what you leave beyond the returned k (hence they are underscores).

Example 2

Input: nums = [0,0,1,1,1,2,2,3,3,4]
Output: 5, nums = [0,1,2,3,4,_,_,_,_,_]
Explanation: Your function should return k = 5, with the first five elements of nums being 0, 1, 2, 3, and 4 respectively.
It does not matter what you leave beyond the returned k (hence they are underscores).

3. Constraints

• 1 <= nums.length <= 3 * $10^4$
• -100 <= nums[i] <= 100
• nums is sorted in non-decreasing order.

4. Solutions

Two Pointers

n = nums.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    int removeDuplicates(vector<int> &nums) {
        int i = 1;
        const int n = nums.size();
        for (int j = 1; j < n; ++j) {
            if (j > i && nums[j] != nums[i - 1]) {
                nums[i] = nums[j];
                ++i;
            }
        }

        return i;
    }
};