feat(work/Easy/Array): add solutions for counting elements and missing number problems

work/Easy/Array/1426.counting-elements.js

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+/*
+ * 1426. Counting Elements
+ * Difficulty: Easy
+ * https://leetcode.com/problems/counting-elements/
+ * 
+ * ──────────────────────────────────────────────────
+ * 
+ * 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.
+ * 
+ *  
+ * 
+ * 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
+*/
+
+/**
+ * @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;
+};