41. First Missing Positive (#102)

Author: ignacio-chiazzo

LeetcodeProblems/Algorithms/hard/First_Missing_Positive.js

@@ -0,0 +1,51 @@
+/*
+41. First Missing Positive
+https://leetcode.com/problems/first-missing-positive/
+Problem:
+Given an unsorted integer array nums. Return the smallest positive integer that is not present in nums.
+You must implement an algorithm that runs in O(n) time and uses O(1) auxiliary space.
+Example 1:
+Input: nums = [1,2,0]
+Output: 3
+Explanation: The numbers in the range [1,2] are all in the array.
+Example 2:
+Input: nums = [3,4,-1,1]
+Output: 2
+Explanation: 1 is in the array but 2 is missing.
+Example 3:
+Input: nums = [7,8,9,11,12]
+Output: 1
+Explanation: The smallest positive integer 1 is missing.
+Constraints:
+1 <= nums.length <= 10^5
+-2^31 <= nums[i] <= 2^31 - 1
+Explanation
+Initialize n
+const n = nums.length;
+This line sets the variable n to the length of the input array nums. It represents the size of the array.
+This is the cyclic sort algorithm. It iterates through the array and, in each step, it checks if the current element nums[i] is within the valid range (1 to n) and not in its correct position. If so, it swaps the element with the one at its correct position.
+After the cyclic sort, this loop searches for the first element that is out of place. If nums[i] is not equal to i + 1, it means that i + 1 is the smallest missing positive integer, and it is returned.
+Return Next Positive Integer if All Elements Are in Place,
+If all elements are in their correct positions, the function returns the next positive integer after the maximum element in the array (n + 1).
+*/
+
+
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var firstMissingPositive = function(nums) {
+  const n = nums.length;
+
+  for (let i = 0; i < n; i++)
+    while (nums[i] > 0 && nums[i] <= n && nums[nums[i] - 1] !== nums[i])
+      [nums[nums[i] - 1], nums[i]] = [nums[i], nums[nums[i] - 1]];
+
+  for (let i = 0; i < n; i++)
+    if (nums[i] !== i + 1)
+      return i + 1;
+
+  return n + 1;
+};
+
+module.exports.firstMissingPositive = firstMissingPositive;

LeetcodeProblemsTests/Algorithms/hard/First_Missing_Positive_Test.js

@@ -0,0 +1,19 @@
+const assert = require("assert");
+const bitReverseToMakeNumberEqual = require("../../../LeetcodeProblems/Algorithms/hard/First_Missing_Positive").firstMissingPositive;
+
+function test() {
+  assert.deepEqual(
+    firstMissingPositive([1,2,0]),
+   3
+  );
+  assert.deepEqual(
+    firstMissingPositive([3,4,-1,1]),
+    2
+  );
+  assert.deepEqual(
+  firstMissingPositive([7,8,9,11,12]),
+  1
+);
+}
+
+module.exports.test = test;

README.md

@@ -16,6 +16,7 @@ The solutions are located under `/LeetcodeProblems`. Each problem has a test fil
 | Name                                                                                                                                                           | Level  | Link                                                                                                                | 
 |----------------------------------------------------------------------------------------------------------------------------------------------------------------|--------|---------------------------------------------------------------------------------------------------------------------|
 | [Edit Distance ](/LeetcodeProblems/Algorithms/hard/Edit_Distance.js)                                                                                           | Hard   | https://leetcode.com/problems/edit-distance/                                                                        |
+| [First Missing Positive ](/LeetcodeProblems/Algorithms/hard/First_Missing_Positive.js)                   | Hard   | https://leetcode.com/problems/first-missing-positive/                                                                        |
 | [Remove Invalid Parentheses ](/LeetcodeProblems/Algorithms/hard/Remove_Invalid_Parentheses.js)                                                                 | Hard   | https://leetcode.com/problems/remove-invalid-parentheses/                                                           |
 | [Longest Consecutive Sequence ](/LeetcodeProblems/Algorithms/hard/Longest_Consecutive_Sequence.js)                                                             | Hard   | https://leetcode.com/problems/longest-consecutive-sequence/                                                         |
 | [Minimum Window Substring ](/LeetcodeProblems/Algorithms/hard/Minimum_Window_Substring.js)                                                                     | Hard   | https://leetcode.com/problems/minimum-window-substring/                                                             |