The answer by  is perfect:

Lesson learnt: always try to dig more insightful thoughts to find hidden\subtle mechanisms.

#include 
     
      #include 
      
       #include 
       
        #include 
        
         #include 
         
          #include 
          
           using namespace std;int main() { // Get input int n; cin >> n; vector
           
             in(n, 0); for(int i = 0; i < n; i ++) cin >> in[i]; // int maxbit = floor(log(*std::max_element(in.begin(), in.end()))/log(2)); int k = -1; for (int bitcnt = maxbit - 1; bitcnt >= 0; bitcnt --) { bool bValid = true; unordered_map
            
              rec; for(auto v : in) { int vs = v >> bitcnt; rec[vs] ++; if(rec[vs] > (n/2)) { bValid = false; break; } } if(!bValid) { continue; } k = bitcnt; break; } cout << k << endl; return 0;}