为什么时间复杂度为O（n ^ 2）而不是O（n ^ 3）？

Question

我解决了a problem on LeetCode，我发现an answer时间复杂度为O（n ^ 2），为什么？这是代码

class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        result = []
        nums.sort()
        for i in xrange(len(nums)-2):
            if i > 0 and nums[i] == nums[i-1]:
                continue
            l, r = i + 1, len(nums) - 1
            while l < r:
                if nums[l] + nums[r] == -nums[i]:
                    result.append([nums[i], nums[l], nums[r]])
                    l, r = l + 1, r - 1
                    while l < r and nums[l] == nums[l-1]:
                        l += 1
                    while l < r and nums[r] == nums[r+1]:
                        r -= 1
                elif nums[l] + nums[r] > -nums[i]:
                    r -= 1
                else:
                    l += 1
        return result