2 Pointer Remove Element

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Unit 4 Session 1 (Click for link to problem statements)

U-nderstand

Understand what the interviewer is asking for by using test cases and questions about the problem.

• What happens to the elements in the array after the specified value is removed?
  • They should be moved to the front of the array in their original order, and the remaining space will not be considered.
• Are there constraints on modifying the array?
  • The operation must be done in-place, and no extra space for another array should be used.

P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Iterate through the array with a fast pointer to examine each element. Use a slow pointer to place elements that are not equal to the target value.

1) Initialize a slow pointer `i` at the start of the array.
2) For each element in the array (`j` as the fast pointer):
  a) If the element at `j` is not equal to `val`:
     i) Place the element at the slow pointer's current position.
    ii) Increment the slow pointer `i`.
3) Return the position of the slow pointer, which indicates the new length of the array.

⚠️ Common Mistakes

• Mismanaging the pointers which can lead to missing elements or incorrect placements.
• Returning the whole list instead of the new length of the relevant part of the list.

I-mplement

def removeElement(nums, val):
    i = 0  # Start from the first element
    for j in range(len(nums)):
        if nums[j] != val:
            nums[i] = nums[j]
            i += 1
    return i