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apps/docs/content/array/1426-counting-elements.mdx

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+---
+title: '1426. Counting Elements'
+description: Given an integer array arr, count how many elements x there are, such that x + 1 is also in arr. If there are duplicates in arr, count them separately
+sidebar:
+  label: 'Counting Elements'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Array</Badge>
+
+### Example 1:
+- Input: `arr = [1,2,3]`
+- Output: `2`
+- Explanation: `1` and `2` are counted cause `2` and `3` are in `arr`.
+
+### Example 2:
+- Input: `arr = [1,1,3,3,5,5,7,7]`
+- Output: `0`
+- Explanation: No numbers are counted, cause there is no `2`, `4`, `6`, or `8` in `arr`.
+
+### Constraints:
+
+- `1 <= arr.length <= 1000`
+- `0 <= arr[i] <= 1000`
+
+## Approach
+
+```mermaid
+flowchart TD
+  S(["countElements(arr)"]) --> I["arrSet = Set(arr), count = 0"]
+  I --> L{"more n in arr?"}
+  L -- no --> E(["return count"])
+  L -- yes --> H{"arrSet has n + 1?"}
+  H -- yes --> C["count++"]
+  C --> L
+  H -- no --> L
+```
+
+## Solution
+
+```js
+/**
+ * @param {number[]} arr
+ * @return {number}
+ */
+var countElements = function(arr) {
+  let arrSet = new Set(arr);
+  let count = 0; 
+
+  for (let n of arr) {
+    let sum = n + 1;
+    if (arrSet.has(sum)) {
+      count++;
+    }
+  }
+  return count;
+};
+```