LeetCode 308. Range Sum Query 2D - Mutable

#include <vector>
#include <iostream>
using namespace std;

/*
  Given a 2D matrix, find the sum of the elements inside the rectangle defined
  by its upper left corner(row1, col1) and lower right corner(row2, col2).
  For exmaple:
  Given matrix = [
  [3, 0, 1, 4, 2],
  [5, 6, 3, 2, 1],
  [1, 2, 0, 1, 5],
  [4, 1, 0, 1, 7],
  [1, 0, 3, 0, 5]]

  sumRegion(2, 1, 4, 3) -> 8
  update(3, 2, 2)
  sumRegion(2, 1, 4, 3) -> 10
  Note:
    The matrix is only modifiable by the update function
    You may assume the number of calls to update and sumRegion function is distributed evenly.
    You may assume that row1 <= row2 and col1 <= col2.
*/
class NumMatrix {
private:
  vector< vector<int> > newMatrix;
public:
  NumMatrix(vector< vector<int> >& matrix) {
    for(int i = 0; i < matrix.size(); ++i) {
      newMatrix.push_back(vector<int>(matrix[0].size() +  1, 0));
    }
    for(int i = 0; i < matrix.size(); ++i) {
      for(int j = 0;  j < matrix[i].size(); ++j) {
        newMatrix[i][j+1] = newMatrix[i][j] + matrix[i][j];
      }
    }
  }
  void update(int row, int col, int val) {
    int diff = val - (newMatrix[row][col + 1] - newMatrix[row][col]);
    for(int i = col + 1; i < newMatrix[row].size(); ++i) {
      newMatrix[row][i] += diff;
    }
  }

  int sumRegion(int row1, int col1, int row2, int col2) {
    int sum = 0;
    for(int i = row1; i <= row2; ++i) {
      sum += newMatrix[i][col2 + 1] - newMatrix[i][col1];
    }
    return sum;
  }
};

int main(void) {
  vector< vector<int> > matrix_test{
  {3, 0, 1, 4, 2},
  {5, 6, 3, 2, 1},
  {1, 2, 0, 1, 5},
  {4, 1, 0, 1, 7},
  {1, 0, 3, 0, 5}};
  NumMatrix matrix(matrix_test);
  cout << matrix.sumRegion(2, 1, 4, 3) << endl;
  matrix.update(3, 2, 2);
  cout << matrix.sumRegion(2, 1, 4, 3) << endl;
}