python leetcode 268. Missing Number

class Solution:
    def missingNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        n=len(nums)
        if n==0:
            return 0 
        i=0
        j=n-1
        while i<j:
            cur=nums[i]
            if cur==i:
                i+=1
            else:
                if cur>=n:
                    nums[i],nums[j]=nums[j],nums[i]
                    j-=1 
                else:
                    nums[i],nums[cur]=nums[cur],nums[i]
        for i in range(n):
            if nums[i]!=i:
                return i 
        return n