Create 1588-sum-of-all-odd-length-subarrays.js

Author: everthis

1588-sum-of-all-odd-length-subarrays.js

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+/**
+ * @param {number[]} arr
+ * @return {number}
+ */
+const sumOddLengthSubarrays = function(arr) {
+  const n = arr.length, pre = Array(n + 1).fill(0)
+  for(let i = 0; i < n; i++) pre[i + 1] = pre[i] +  arr[i]
+  
+  let res = 0
+  let len = 1
+  while(len <= n) {
+    for(let i = 0; i <= n - len; i++) {
+      res += pre[i + len] - pre[i] // len === 1: 1 - 0, 2 - 1
+                                   // len === 3: 3 - 0, 6 - 3
+    }
+    
+    len += 2
+  }
+  
+  return res
+  
+};