LeetCode/137. Single Number II (Bit Manipulation) at master · BrianSong/LeetCode · GitHub

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137. Single Number II
Medium

751

244

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Given a non-empty array of integers, every element appears three times except for one, which appears exactly once. Find that single one.

Note:

Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

Example 1:

Input: [2,2,3,2]
Output: 3
Example 2:

Input: [0,1,0,1,0,1,99]
Output: 99

Solution:------------------------------------ Bit Manipulation ------- https://www.youtube.com/watch?v=puXcQpwgcD0
--------------------------------------------- O(n) T, O(1) S
'''
& : and
| : or
^ : XOR, same -> 0, different -> 1
~ : 0 -> 1, 1 -> 0
<<: 左移
>>: 右移
'''
class Solution(object):
    def singleNumber(self, nums):
        if not nums:
            return None
        res = 0
        for i in range(32):
            counter = 0
            for num in nums:
                counter += (num >> i) & 1     # >> : 右移动运算符：把">>"左边的运算数的各二进位全部右移若干位，>> 右边的数字指定了移动的位数
                                              # num >> i is an integer, we use & 1 to get the binary_representation[i]
            rem = counter % 3                 # take the remainder with 3 because every element appears three times except for one
                                              # So, if every element appears five times except for one, we use % 5
            if i == 31 and rem:               # Must handle the negative case, and IMPORTANT !!!!!!! we must let rem = counter % 3
                                              # before we can handle the negative case, otherwise will bring error !!!!!
                                              # because the rem is meaningless before we do % 3,
                                              # so the negative case cannot be proceeded before we get the meaningful rem
                res -= 1 << 31                # 1 << 31 = 2147483648
            else:
                res |= rem << i               # use |= to update the res in binary form everytime
        return res
        """
        :type nums: List[int]
        :rtype: int
        """