137. Single Number II

https://leetcode.com/problems/single-number-ii/

Bitwise operators

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class Solution {
  public int singleNumber(int[] nums) {
    int once = 0, twice = 0;
    for (int num : nums) {
      once = (~twice) & (once ^ num);
      twice = (~once) & (twice ^ num);
    }
      return once;
  }
}