这是一个 O(N) 解决方案吗？我被告知这是 O(N^2) 因为 in 关键字

Question

我相信下面的代码是一个 O(N) 解决方案，但我被告知它是 O(N^2)，因为 in 关键字。这是真的吗？

代码：

class Solution:
    def twoSum(self, nums, target):
        for i,v in enumerate(nums):
            value = target - v
            if value in nums and nums.index(value)!= I:
                return (nums.index(value),I)
                break

Answer 1

x in a_list 和 nums.index(value) 一样是 O(n) ...所以你的内部 if 语句是 O(2n) 而你的外部 for 循环是 O(n)

所以你有 O(n)*O(2n) = O(2n^2) = O(n^2)

O(n)解如下

def twoSum(x,targetValue):
    # pending values is a dict of index's mapped where the key is key + x[pendingValues[key]] = targetValue
    pendingValues = {}
    for i,value in enumerate(x):
        if value in pendingValues: # in Dict is only O(1)
           # this is a match return both matching index's
           return (pendingValues[value],i)
        else:
           # not a match store the index as matchedTarget=index
           pendingValues[targetValue-value] = i