Create 0041-first-missing-positive.js

Added Solution for first-missing-positive  in JS

Author: aadil42

javascript/0041-first-missing-positive.js

@@ -0,0 +1,47 @@
+ /**
+   * Time O(n) | Space O(1)
+ * @param {number[]} nums
+ * @return {number}
+ */
+var firstMissingPositive = function(nums) {
+    //-1 1 3 4
+    const swap = (a,b) => {
+        const temp = nums[a];
+        nums[a] = nums[b];
+        nums[b] = temp;
+    }
+    // swap positions
+    for(let i = 0; i < nums.length; i++) {
+        const el = nums[i];
+        const chair = el - 1;
+        if(el >= 1 &&  el <= nums.length + 1 && nums[chair] !== el) {
+             swap(chair, i);
+                i--; // this decrement is important // check this input [3,4,-1,1]
+        }
+    }
+
+
+    for(let i = 0; i < nums.length; i++) {
+        if(nums[i] !== i+1) return i+1;
+    } 
+
+    return nums.length + 1;
+};
+
+/**
+ * https://leetcode.com/problems/first-missing-positive/
+ * Time O(n) | Space O(n)
+ * @param {number[]} nums
+ * @return {number}
+ */
+var firstMissingPositive1 = function(nums) {
+
+    const numberSet = new Set(nums);
+      
+      let i = 1;
+      while(numberSet.has(i)) {
+          i++;
+      }
+  
+      return i;
+  };