Missing Number solution

Author: kim-young

missing-number/kimyoung.js

@@ -0,0 +1,30 @@
+var missingNumber = function (nums) { // brute force approach
+    let missingNumber;
+    for (let i = 0; i < nums.length; i++) {
+        if (nums.indexOf(i) === -1) missingNumber = i;
+    }
+    return missingNumber === undefined ? nums.length : missingNumber;
+};
+
+// time - O(n^2) finding the index of i while iterating through nums
+// splace - O(1) no extra space needed other than missingNumber which just stores the result
+
+var missingNumber = function (nums) { // set approach 
+    let set = new Set(nums);
+    for (let i = 0; i < nums.length + 1; i++) {
+        if (!set.has(i)) return i;
+    }
+};
+
+// time - O(n) looping through nums to find the missing number
+// splace - O(n) creating a set of nums
+
+var missingNumber = function (nums) { // mathematical approach
+    const len = nums.length
+    const expectedSum = len * (len + 1) / 2;
+    const actualSum = nums.reduce((acc, el) => acc += el, 0);
+    return expectedSum - actualSum;
+};
+
+// time - O(n) reduce method on actualSum
+// space - O(1) extra space irrelevant to input