solution for the first missing positive

Author: emikhalev

leetcode/first_missing_positive-solution-1-sort.go

@@ -0,0 +1,51 @@
+// https://leetcode.com/submissions/detail/445082440/
+
+package main
+
+func firstMissingPositive(nums []int) int {
+sort.Slice(nums, func(i, j int) bool { return nums[i] < nums[j] })
+
+expectedNum := 1
+for i := 0; i < len(nums); i++ {
+if nums[i] <= 0 {
+continue
+}
+
+if nums[i] != expectedNum {
+break
+}
+
+for ; i < len(nums) && nums[i] == expectedNum; i++ {
+}
+i--
+expectedNum++
+}
+
+return expectedNum
+}
+
+// 1, 2, 0 -> 0, 1, 2
+// n=0, exp=0, c; n=1, exp=0, exp=2(n+1), c; n=2, exp=2, exp=3(++); c; eoc; ret exp(3)
+
+// 3, 4, -1, 1 -> -1, 1, 3, 4 -> 1
+// n=-1, c; n=1, exp=2; n=3, exp=2, break; return exp(2)
+
+// Trivial:
+// 1. sort - N*logN CPU, O(N) - space complexity for merge sort, qsort
+// 2. iterate - find first positive, after that find first missing positive
+
+// Test cases
+// 1. simple: [0, 1, 2, 3, 5], [34, 36, 50], [-50, -30, -3, -2, -1, 0, 1, 2, 3, 5]
+// 2. missing two or more positives: [0, 1, 2, 3, 5, 8, 12]
+// 3. no-zero: [[1, 2, 3, 5]]
+// 4. no-zero and negatives: [-1, -2, -4, 1, 2, 3, 5]
+// 5. empty: []
+// 6. no-positives: [-5, -4, -3, -2, -1]
+// 7. no-positives and zero: [-5, -4, -3, -2, -1, 0]
+// 8. no-positives, zero and missing negatives: [-10, -8, -5, -4, -3, -2, -1, 0]
+// 9. no missing positives: 2^31-1 [1<<31-3, 1<<31-2, 1<<31-1] -> [2147483645, 2147483646, 2147483647]
+// 10. [0]
+// 11. [1]
+// 12. [2]
+// 13. [-2]
+// 14. Repeatable: [0,2,2,1,1]