27. Remove Element

Edited by Charles

Easy · Python · Array

Problem

Read the original problem on LeetCode.

The official page contains the problem statement, examples, and constraints.

Explanation, examples & other approaches

Core idea

Dual Pointers from the Start: Consider using two pointers that both start from the beginning of the array. Element Overwriting: The goal is to keep only the elements that are not equal to val. This means we are not "swapping" elements, but rather "overwriting" them. Elements equal to val will be covered (overwritten) by subsequent elements that are not equal to val.

Original study notes follow, including intermediate snippets and earlier attempts.

LeetCode 27. Remove Element

1. Dual Pointers from the Start: Consider using two pointers that both start from the beginning of the array.
2. Element Overwriting: The goal is to keep only the elements that are not equal to val. This means we are not "swapping" elements, but rather "overwriting" them. Elements equal to val will be covered (overwritten) by subsequent elements that are not equal to val.
3. Pointer Roles:
   • 3.1 Fast Pointer (f): Moves forward at its own pace using a for loop.
   • 3.2 Slow Pointer (s): Has a specific function: whenever the fast pointer points to an element that is not val, the slow pointer overwrites its current position with that element and then moves one step forward.
     • (1) When the fast and slow pointers are synchronized, the overwriting simply replaces the value with itself (Splendid!).
     • (2) When the fast pointer encounters val, the slow pointer stays put. This is because the slow pointer is now pointing at an element that needs to be overwritten. The fast pointer continues to move forward to find the next element that is not val.
4. Result Interpretation: Since Python uses 0-based indexing, the final value of the slow pointer represents the total count of elements not equal to val. These elements have been stored in the array from index 0 to s-1.

Study record

Field	Value
Attempts	4
Mastery	❚❚❚❚
The Latest Attempt	December 31, 2025
Feedback	一开始想到了双指针，但是总是想前后双指针解决问题，同时起步的双指针要多练习！

Python solution

class Solution:
    def removeElement(self, nums: List[int], val: int) -> int:
        n = len(nums)
        s = 0

        for f in range(n):
            # nums[i] == val
            #   pass and let record stay
            # nums[i] != val
            #   replace and let record iterate
            if nums[f] != val:
                nums[s] = nums[f]
                s += 1
        return s

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