LeetCode: 421. Maximum XOR of Two Numbers in an Array

The problem is https://leetcode.com/problems/maximum-xor-of-two-numbers-in-an-array/. If just follow the process defined as described in problem, it is quite simple except the runtime is O(n**2):

class Solution(object):
    def findMaximumXOR(self, nums):
        :type nums: List[int]
        :rtype: int
        if len(nums)<2:
            return 0
        ret = nums[0]^nums[1]
        for i in range(len(nums)-1):
            for j in range(i+1, len(nums)):
                tmp = nums[i]^nums[j]
                if tmp > ret:
                    ret = tmp
        return ret

And as expected, the result is "time limit exceeded".