Create 0041-first-missing-positive.kt

Author: a93a

Diff for: kotlin/0041-first-missing-positive.kt

@@ -0,0 +1,20 @@
+class Solution {
+    fun firstMissingPositive(nums: IntArray): Int {
+        var i = 0
+        // if we encounter a valid integer, swap it into its right place. Ignore all numbers larger than nums.length
+        while(i < nums.size) {
+            val pos = nums[i] - 1
+            if(pos in (0 until nums.size) && nums[i] != nums[pos]) nums.swap(i, pos)
+            else i++
+        }
+
+        //if the expected number is not in its right place, return it
+        for(i in nums.indices){
+            if (nums[i] != i + 1) return i + 1
+        }
+        return nums.size + 1
+    }
+    private fun IntArray.swap(i: Int, j: Int) {
+        this[i] = this[j].also{ this[j] = this[i]}
+    }
+}