add: FindFirstMissingPositive rehearsal with in-place O(n) solution

array/find_first_missing_positive.go

@@ -0,0 +1,21 @@
+package array
+
+// FindFirstMissingPositive returns the smallest missing positive integer.
+// The algorithm runs in O(n) time and O(1) space by placing each value at its correct index.
+func FindFirstMissingPositive(nums []int) int {
+	n := len(nums)
+	for i := 0; i < n; i++ {
+		for nums[i] > 0 && nums[i] <= n && nums[nums[i]-1] != nums[i] {
+			// Swap nums[i] with nums[nums[i] - 1]
+			nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]
+		}
+	}
+
+	for i := 0; i < n; i++ {
+		if nums[i] != i+1 {
+			return i + 1
+		}
+	}
+	return n + 1
+}
+

array/find_first_missing_positive_test.go

@@ -0,0 +1,42 @@
+package array
+
+import (
+	"testing"
+)
+
+/*
+TestFindFirstMissingPositive tests solution(s) with the following signature:
+
+	FindFirstMissingPositive(nums []int) int
+
+Given an unsorted integer slice, return the smallest missing positive integer.
+
+Examples:
+Input: []int{1, 2, 0}           → Output: 3
+Input: []int{3, 4, -1, 1}       → Output: 2
+Input: []int{7, 8, 9, 11, 12}   → Output: 1
+*/
+func TestFindFirstMissingPositive(t *testing.T) {
+	tests := []struct {
+		nums     []int
+		expected int
+	}{
+		{[]int{1, 2, 0}, 3},
+		{[]int{3, 4, -1, 1}, 2},
+		{[]int{7, 8, 9, 11, 12}, 1},
+		{[]int{}, 1},
+		{[]int{1, 2, 3}, 4},
+		{[]int{0, -1, -2}, 1},
+		{[]int{2, 1}, 3},
+		{[]int{1}, 2},
+		{[]int{2}, 1},
+		{[]int{3, 4, -1, 1, 2}, 5},
+	}
+
+	for i, test := range tests {
+		if got := FindFirstMissingPositive(append([]int(nil), test.nums...)); got != test.expected {
+			t.Fatalf("Failed test case #%d. Input: %v. Want %d, got %d", i, test.nums, test.expected, got)
+		}
+	}
+}
+