存在重复元素

使用哈希表（字典），出现访问错误会报KeyError，若未出现即返回True，否则返回False，复杂度O(N)

class Solution:
    def containsDuplicate(self, nums: List[int]) -> bool:
        visited = {}
        for num in nums:
            try:
                temp = visited[num]
                return True
            except KeyError:
                visited[num] = 'h'

        return False

或者：

class Solution:
    def containsDuplicate(self, nums: List[int]) -> bool:
        visited = {}
        for num in nums:
            if num in visited:
                return True
            else:
                visited[num] = 'h'

        return False

或使用set：

class Solution:
    def containsDuplicate(self, nums: List[int]) -> bool:
        
        return len(nums) != len(set(nums))