[LeetCode] Range Sum Query 2D - Immutable

Very similar to Range Sum Query - Immutable, but we now need to compute a 2d accunulated-sum. In fact, if you work in computer vision, you may know a name for such an array --- Integral Image.

To solve this problem, Stefan has already posted a very elegant solution.

The code is copied here.

 class NumMatrix {

 public:

     NumMatrix(vector<vector<int>> &matrix) {

         accu = matrix;

         for (int i = ; i < matrix.size(); i++)

             for (int j = ; j < matrix[].size(); j++)

                 accu[i][j] += a(i-, j) + a(i, j-) - a(i-, j-);

     }

     int sumRegion(int row1, int col1, int row2, int col2) {

         return a(row2, col2) - a(row1-, col2) - a(row2, col1-) + a(row1-, col1-);

     }

 private:

     vector<vector<int>> accu;

     int a(int i, int j) {

         return i >=  && j >=  ? accu[i][j] : ;

     }

 };

 // Your NumMatrix object will be instantiated and called as such:

 // NumMatrix numMatrix(matrix);

 // numMatrix.sumRegion(0, 1, 2, 3);

 // numMatrix.sumRegion(1, 2, 3, 4);