303 前序和

Author: tbswang

303.ts

@@ -0,0 +1,41 @@
+/*
+ * @lc app=leetcode.cn id=303 lang=typescript
+ *
+ * [303] 区域和检索 - 数组不可变
+ */
+
+// @lc code=start
+class NumArray {
+    sum = []
+    constructor(nums: number[]) {
+        let init = 0;
+        // sum:0,0; sum:1,  num[0], sum:1, num{0 + 1}
+        this.sum = new Array(nums.length + 1).fill(0)
+        nums.map((n, idx) => {
+            this.sum[idx + 1] = init + n
+            init = this.sum[idx + 1]
+        })
+        console.log(this.sum)
+
+    }
+
+    sumRange(left: number, right: number): number {
+        // sum[2] - sum[0] = [0,1,2] - 0
+        // 2-5 = sum[5]- sum[2] = [0,1,2,3,4,] - [0,1,2,] = [ 3,4]
+        // 
+        return this.sum[right + 1] - this.sum[left]
+    }
+}
+
+/**
+ * Your NumArray object will be instantiated and called as such:
+ * var obj = new NumArray(nums)
+ * var param_1 = obj.sumRange(left,right)
+ */
+// @lc code=end
+
+
+let numArray = new NumArray([-2, 0, 3, -5, 2, -1]);
+console.log(numArray.sumRange(0, 2)); // return 1 ((-2) + 0 + 3)
+console.log(numArray.sumRange(2, 5)); // return -1 (3 + (-5) + 2 + (-1)) 
+console.log(numArray.sumRange(0, 5)); // return -3 ((-2) + 0 + 3 + (-5) + 2 + (-1))