How can you eliminate duplicate elements in a sorted array without resorting to re-sorting?

If the array is already sorted, then duplicate elements will be adjacent. To remove duplicates without re-sorting, you can iterate through the array once and copy unique elements into a new array or modify the original array in place using two pointers.

Example in Java with in-place modification (for an int array):

public static int removeDuplicates(int[] nums) {
    if (nums.length == 0) return 0;
    int uniqueIndex = 0;
    for (int i = 1; i < nums.length; i++) {
        if (nums[i] != nums[uniqueIndex]) {
            uniqueIndex++;
            nums[uniqueIndex] = nums[i];
        }
    }
    return uniqueIndex + 1; // new length of the array without duplicates
}

In the end, the first uniqueIndex + 1 elements of the array will be unique. This approach works in O(n) time and does not require additional sorting.