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137. Single Number II 👍

Approach 1: Bit

  • Time: $O(32n) = O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  int singleNumber(vector<int>& nums) {
    int ans = 0;

    for (int i = 0; i < 32; ++i) {
      int sum = 0;
      for (const int num : nums)
        sum += num >> i & 1;
      sum %= 3;
      ans |= sum << i;
    }

    return ans;
  }
};

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class Solution {
  public int singleNumber(int[] nums) {
    int ans = 0;

    for (int i = 0; i < 32; ++i) {
      int sum = 0;
      for (final int num : nums)
        sum += num >> i & 1;
      sum %= 3;
      ans |= sum << i;
    }

    return ans;
  }
}

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class Solution:
  def singleNumber(self, nums: list[int]) -> int:
    ones = 0
    twos = 0

    for num in nums:
      ones ^= num & ~twos
      twos ^= num & ~ones

    return ones

Approach 2: Magic

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  int singleNumber(vector<int>& nums) {
    int ones = 0;
    int twos = 0;

    for (const int num : nums) {
      ones ^= (num & ~twos);
      twos ^= (num & ~ones);
    }

    return ones;
  }
};

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class Solution {
  public int singleNumber(int[] nums) {
    int ones = 0;
    int twos = 0;

    for (final int num : nums) {
      ones ^= (num & ~twos);
      twos ^= (num & ~ones);
    }

    return ones;
  }
}

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class Solution:
  def singleNumber(self, nums: list[int]) -> int:
    ones = 0
    twos = 0

    for num in nums:
      ones ^= (num & ~twos)
      twos ^= (num & ~ones)

    return ones