text: #
  # @lc app=leetcode.cn id=268 lang=python3
  #
  # [268] 缺失数字
  #
  # https://leetcode-cn.com/problems/missing-number/description/
  #
  # algorithms
  # Easy (48.30%)
  # Total Accepted:    14.9K
  # Total Submissions: 30.7K
  # Testcase Example:  '[3,0,1]'
  #
  # 给定一个包含 0, 1, 2, ..., n 中 n 个数的序列，找出 0 .. n 中没有出现在序列中的那个数。
  # 
  # 示例 1:
  # 
  # 输入: [3,0,1]
  # 输出: 2
  # 
  # 
  # 示例 2:
  # 
  # 输入: [9,6,4,2,3,5,7,0,1]
  # 输出: 8
  # 
  # 
  # 说明:
  # 你的算法应具有线性时间复杂度。你能否仅使用额外常数空间来实现?
  # 
  #
  class Solution:
      def missingNumber(self, nums: List[int]) -> int:
          length = len(nums)
          if length == 0:
              return 0
          
          sum_nums = sum(nums)
          
          return int(length*(length+1) / 2) - sum(nums)
  
  