feat: solve No.303

Author: BaffinLee

301-400/303. Range Sum Query - Immutable.md

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+# 303. Range Sum Query - Immutable
+
+- Difficulty: Easy.
+- Related Topics: Array, Design, Prefix Sum.
+- Similar Questions: Range Sum Query 2D - Immutable, Range Sum Query - Mutable, Maximum Size Subarray Sum Equals k.
+
+## Problem
+
+Given an integer array `nums`, handle multiple queries of the following type:
+
+
+	
+- Calculate the **sum** of the elements of `nums` between indices `left` and `right` **inclusive** where `left <= right`.
+
+
+Implement the `NumArray` class:
+
+
+	
+- `NumArray(int[] nums)` Initializes the object with the integer array `nums`.
+	
+- `int sumRange(int left, int right)` Returns the **sum** of the elements of `nums` between indices `left` and `right` **inclusive** (i.e. `nums[left] + nums[left + 1] + ... + nums[right]`).
+
+
+ 
+Example 1:
+
+```
+Input
+["NumArray", "sumRange", "sumRange", "sumRange"]
+[[[-2, 0, 3, -5, 2, -1]], [0, 2], [2, 5], [0, 5]]
+Output
+[null, 1, -1, -3]
+
+Explanation
+NumArray numArray = new NumArray([-2, 0, 3, -5, 2, -1]);
+numArray.sumRange(0, 2); // return (-2) + 0 + 3 = 1
+numArray.sumRange(2, 5); // return 3 + (-5) + 2 + (-1) = -1
+numArray.sumRange(0, 5); // return (-2) + 0 + 3 + (-5) + 2 + (-1) = -3
+```
+
+ 
+**Constraints:**
+
+
+	
+- `1 <= nums.length <= 104`
+	
+- `-105 <= nums[i] <= 105`
+	
+- `0 <= left <= right < nums.length`
+	
+- At most `104` calls will be made to `sumRange`.
+
+
+
+## Solution
+
+```javascript
+/**
+ * @param {number[]} nums
+ */
+var NumArray = function(nums) {
+    this.leftSum = Array(nums.length);
+    for (var i = 0; i < nums.length; i++) {
+        this.leftSum[i] = (this.leftSum[i - 1] || 0) + nums[i];
+    }
+};
+
+/** 
+ * @param {number} left 
+ * @param {number} right
+ * @return {number}
+ */
+NumArray.prototype.sumRange = function(left, right) {
+    return this.leftSum[right] - (this.leftSum[left - 1] || 0);
+};
+
+/** 
+ * Your NumArray object will be instantiated and called as such:
+ * var obj = new NumArray(nums)
+ * var param_1 = obj.sumRange(left,right)
+ */
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(1).
+* Space complexity : O(n).