下面代码用分治求“最大连续子段和”，其时间复杂度为（ ）。

1int solve(vector<int>& a, int l, int r){                              2   if(l == r) return a[l];                                            3                                                                      4   int mid = l + (r - l) / 2;                                         5                                                                      6   int left = solve(a, l, mid);                                       7   int right = solve(a, mid + 1, r);                                  8                                                                      9   int sum = 0, lmax = INT_MIN;                                       10   for(int i = mid; i >= l; i--){                                     11      sum += a[i];                                                    12      lmax = max(lmax, sum);                                          13   }                                                                  14                                                                      15   sum = 0;                                                           16   int rmax = INT_MIN;                                                17   for(int i = mid + 1; i <= r; i++){                                 18      sum += a[i];                                                    19      rmax = max(rmax, sum);                                          20   }                                                                  21                                                                      22   return max({left, right, lmax + rmax});                            23}