27. Remove Element

Given an integer array nums and an integer val, remove all occurrences of val in nums in-place. The order of the elements may be changed. Then return the number of elements in nums which are not equal to val.

Consider the number of elements in nums which are not equal to val be k, to get accepted, you need to do the following things:

• Change the array nums such that the first k elements of nums contain the elements which are not equal to val. The remaining elements of nums are not important as well as the size of nums.
• Return k.

Example 1:

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

Example 2:

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

Explanation: Slow-Fast Pointer Approach

✅ Idea:

The slow-fast pointer technique is used to iterate through an array and selectively overwrite values in-place.

• The fast pointer scans each element of the array.
• The slow pointer keeps track of where the next non-val element should be placed.

🔍 Logic:

1. Start both pointers at the beginning of the array.
2. Move the fast pointer one step at a time.
3. If nums[fast] is not equal to val, assign nums[slow] = nums[fast], then move slow forward.
4. At the end, slow will represent the length of the filtered array.

🎯 Goal:

• Remove all occurrences of val in-place.
• Return the new length k (i.e., number of elements not equal to val).

💡 Key Point:

This method preserves the relative order of elements that are not equal to val.

Implementations

🔹 Go

func removeElement(nums []int, val int) int {
    slow := 0
    for fast := 0; fast < len(nums); fast++ {
        if nums[fast] != val {
            nums[slow] = nums[fast]
            slow++
        }
    }
    return slow
}

🔸 PHP

function removeElement(&$nums, $val) {
    $slow = 0;
    for ($fast = 0; $fast < count($nums); $fast++) {
        if ($nums[$fast] !== $val) {
            $nums[$slow] = $nums[$fast];
            $slow++;
        }
    }
    return $slow;
}

🔹 C#

public class Solution {
    public int RemoveElement(int[] nums, int val) {
        int slow = 0;
        for (int fast = 0; fast < nums.Length; fast++) {
            if (nums[fast] != val) {
                nums[slow] = nums[fast];
                slow++;
            }
        }
        return slow;
    }
}