fn sum_region(&self, row1: i32, col1: i32, row2: i32, col2: i32) -> i32 {
let row1 = row1 as usize; let col1 = col1 as usize; let row2 = row2 as usize; let col2 = col2 as usize;
return self.dp[row2 + 1][col2 + 1] - self.dp[row1][col2 + 1] - self.dp[row2 + 1][col1] + (self.dp[row1][col1]);
}