Add solution #304

Author: JoshCrozier

README.md

@@ -248,6 +248,7 @@
 300|[Longest Increasing Subsequence](./0300-longest-increasing-subsequence.js)|Medium|
 301|[Remove Invalid Parentheses](./0301-remove-invalid-parentheses.js)|Hard|
 303|[Range Sum Query - Immutable](./0303-range-sum-query-immutable.js)|Easy|
+304|[Range Sum Query 2D - Immutable](./0304-range-sum-query-2d-immutable.js)|Medium|
 306|[Additive Number](./0306-additive-number.js)|Medium|
 309|[Best Time to Buy and Sell Stock with Cooldown](./0309-best-time-to-buy-and-sell-stock-with-cooldown.js)|Medium|
 315|[Count of Smaller Numbers After Self](./0315-count-of-smaller-numbers-after-self.js)|Hard|

solutions/0304-range-sum-query-2d-immutable.js

@@ -0,0 +1,45 @@
+/**
+ * 304. Range Sum Query 2D - Immutable
+ * https://leetcode.com/problems/range-sum-query-2d-immutable/
+ * Difficulty: Medium
+ *
+ * Given a 2D matrix matrix, handle multiple queries of the following type:
+ * - Calculate the sum of the elements of matrix inside the rectangle defined by its upper
+ *   left corner (row1, col1) and lower right corner (row2, col2).
+ *
+ * Implement the NumMatrix class:
+ * - NumMatrix(int[][] matrix) Initializes the object with the integer matrix matrix.
+ * - int sumRegion(int row1, int col1, int row2, int col2) Returns the sum of the elements
+ *   of matrix inside the rectangle defined by its upper left corner (row1, col1) and lower
+ *   right corner (row2, col2).
+ *
+ * You must design an algorithm where sumRegion works on O(1) time complexity.
+ */
+
+/**
+ * @param {number[][]} matrix
+ */
+var NumMatrix = function(matrix) {
+  this.sums = new Array(matrix.length + 1).fill().map(() => {
+    return new Array(matrix[0].length + 1).fill(0);
+  });
+
+  for (let i = 1; i <= matrix.length; i++) {
+    for (let j = 1; j <= matrix[0].length; j++) {
+      this.sums[i][j] = matrix[i - 1][j - 1] + this.sums[i - 1][j]
+        + this.sums[i][j - 1] - this.sums[i - 1][j - 1];
+    }
+  }
+};
+
+/**
+ * @param {number} row1
+ * @param {number} col1
+ * @param {number} row2
+ * @param {number} col2
+ * @return {number}
+ */
+NumMatrix.prototype.sumRegion = function(row1, col1, row2, col2) {
+  return this.sums[row2 + 1][col2 + 1] - this.sums[row2 + 1][col1]
+    - this.sums[row1][col2 + 1] + this.sums[row1][col1];
+};