quadtree_update.cpp

int update(Node* curr, int row1, int row2, int col1, int col2, int row, int col, int val)
{
    if(row1 == row2 && col1 == col2)
    {
        int add = val - curr->sum;
        curr->sum += add;
        return add;
    }
    int add = 0, rMid = row1 + (row2 - row1) / 2, cMid = col1 + (col2 - col1) / 2;
    if(row <= rMid && col <= cMid)add = update(curr->children[0], row1, rMid, col1, cMid, row, col, val);
    else if(row <= rMid && col > cMid)add = update(curr->children[1], row1, rMid, cMid + 1, col2, row, col, val);
    else if(row > rMid && col <= cMid)add = update(curr->children[2], rMid + 1, row2, col1, cMid, row, col, val);
    else add = update(curr->children[3], rMid + 1, row2, cMid + 1, col2, row, col, val);
    curr->sum += add;
    return add;
}