[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);