Sort Integers by the Number of 1 Bits Problem & Solution

#pragma GCC optimize("Ofast")
#pragma GCC optimization("fast-math")

static const int _=[](){ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

class Solution {
public:
  vector<int> sortByBits(vector<int>& arr) {
    sort(arr.begin(), arr.end(), [](int a, int b) {
      int abits = __builtin_popcount(a);
      int bbits = __builtin_popcount(b);
      if (abits < bbits) {
        return true;
      } else if (abits > bbits) {
        return false;
      } else {
        return a < b;
      }
    });

    return arr;
  }
};