some more problems solved

range-sum-query-2d-immutable/sol.rs

@@ -0,0 +1,41 @@
+struct NumMatrix {
+    sums_matrix: Vec<Vec<i32>>
+}
+
+
+/** 
+ * `&self` means the method takes an immutable reference.
+ * If you need a mutable reference, change it to `&mut self` instead.
+ */
+impl NumMatrix {
+    // Generate an accumulating sum matrix in-place
+    // O(1) extra memory
+    fn new(mut matrix: Vec<Vec<i32>>) -> Self {
+        for row in (0..matrix.len()) {
+            let mut acc = 0;
+            for col in (0..matrix[0].len()) {
+                acc += matrix[row][col];
+                matrix[row][col] = acc;
+                if row != 0 {
+                    matrix[row][col] += matrix[row-1][col];
+                }
+            }
+        }
+        NumMatrix { sums_matrix: matrix }
+    }
+    
+    // Use the accumulating sums to get the sum of the region
+    // Space: O(1), Time: O(1)
+    fn sum_region(&self, row1: i32, col1: i32, row2: i32, col2: i32) -> i32 {
+        let top_rec = if row1 == 0 { 0 } else { self.sums_matrix[row1 as usize - 1][col2 as usize] };
+        let left_rec = if col1 == 0 { 0 } else { self.sums_matrix[row2 as usize][col1 as usize - 1] };
+        let rec_overlap = if row1 == 0 || col1 == 0 { 0 } else { self.sums_matrix[row1 as usize - 1][col1 as usize - 1] };
+        self.sums_matrix[row2 as usize][col2 as usize] - top_rec - left_rec + rec_overlap
+    }
+}
+
+/**
+ * Your NumMatrix object will be instantiated and called as such:
+ * let obj = NumMatrix::new(matrix);
+ * let ret_1: i32 = obj.sum_region(row1, col1, row2, col2);
+ */