lc27

class Solution:
    def removeElement(self, nums: List[int], val: int) -> int:
        if len(nums) == 0:
            return 0
        s, f = 0, 0
        while f < len(nums):
            # 快指针把非val丢过来
            if nums[f] != val:
                nums[s] = nums[f]
                s += 1
            f += 1
        return s