A number that appears only once II

Topic link
Title Description ：
Give you an array of integers nums , Except that one element only appears once Outside , Every other element just happens to appear Three times . Please find and return the element that only appears once

Method 1： Hash

Ideas ：

1. Create a number to map the number of times
2. The number of iterations is 1 Of

Method 2： Bit operation

Ideas ：

1. With [2,2,3,2] For example ：
2. Because other numbers are 3 individual , Fill in the sum of each binary bit of each number 32 Array of elements
3. Traverse %3, That's the number
   

The code is as follows ：

class Solution {
     
public:
   int singleNumber(vector<int>& nums) {
        
       vector<int> bit_arr(32,0);
       for(int i=0;i<nums.size();i++)
       {
     
           int bit=32;
           while(bit--)
           {
     
               bit_arr[bit]+= (1&(nums[i]>>bit));
           }
       }

       int ret=0;
       for(int i=0;i<32;i++)
       {
     
           if(bit_arr[i]%3==1)
           {
     
               ret|=(1<<i);// For example, move left 3 position  0000001000, Press the bit or achieve the effect 
           }
       }
       return ret;
   }
};

Method 3： logical circuit

Reference resources ： Official problem solving ideas

Method 4： Finite state automata

Reference resources ： Krahets How to solve the problem