From 4371f032552be60c5af6f9f91d1bbfe0f031de0d Mon Sep 17 00:00:00 2001
From: "go-interview-practice-bot[bot]"
 <230190823+go-interview-practice-bot[bot]@users.noreply.github.com>
Date: Thu, 6 Nov 2025 12:26:13 +0000
Subject: [PATCH] Add solution for Challenge 21

---
 .../submissions/AlexO-85/solution-template.go | 148 ++++++++++++++++++
 1 file changed, 148 insertions(+)
 create mode 100644 challenge-21/submissions/AlexO-85/solution-template.go

diff --git a/challenge-21/submissions/AlexO-85/solution-template.go b/challenge-21/submissions/AlexO-85/solution-template.go
new file mode 100644
index 00000000..6621b18d
--- /dev/null
+++ b/challenge-21/submissions/AlexO-85/solution-template.go
@@ -0,0 +1,148 @@
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	// Example sorted array for testing
+	arr := []int{1, 3, 5, 7, 9, 11, 13, 15, 17, 19}
+
+	// Test binary search
+	target := 7
+	index := BinarySearch(arr, target)
+	fmt.Printf("BinarySearch: %d found at index %d\n", target, index)
+
+	// Test recursive binary search
+	recursiveIndex := BinarySearchRecursive(arr, target, 0, len(arr)-1)
+	fmt.Printf("BinarySearchRecursive: %d found at index %d\n", target, recursiveIndex)
+
+	// Test find insert position
+	insertTarget := 8
+	insertPos := FindInsertPosition(arr, insertTarget)
+	fmt.Printf("FindInsertPosition: %d should be inserted at index %d\n", insertTarget, insertPos)
+
+	arr = []int{5}
+	index = BinarySearch(arr, target)
+	fmt.Printf("BinarySearch: %d not found. Return value: %d \n", target, index)
+
+}
+
+// BinarySearch performs a standard binary search to find the target in the sorted array.
+// Returns the index of the target if found, or -1 if not found.
+func BinarySearch(arr []int, target int) int {
+	left := 0
+	right := len(arr) - 1
+
+	if right == -1 || target < arr[left] || target > arr[right] {
+		return -1
+	}
+
+	if arr[left] == target {
+		return left
+	}
+
+	if arr[right] == target {
+		return right
+	}
+
+	for left <= right {
+		mid := left + (right-left)/2
+
+		// fmt.Printf("Looking between %d and %d for %d, mid: %d\n", left, right, target, mid)
+
+		if mid == left || mid == right {
+			return -1
+		}
+
+		switch {
+		case target < arr[mid]:
+			right = mid
+		case target > arr[mid]:
+			left = mid
+		default:
+			return mid
+		}
+
+	}
+
+	return -1
+}
+
+// BinarySearchRecursive performs binary search using recursion.
+// Returns the index of the target if found, or -1 if not found.
+func BinarySearchRecursive(arr []int, target int, left int, right int) int {
+
+	if right == -1 || left > right || target < arr[left] || target > arr[right] {
+		return -1
+	}
+
+	if arr[left] == target {
+		return left
+	}
+
+	if arr[right] == target {
+		return right
+	}
+
+	mid := left + (right-left)/2
+
+	if mid == left || mid == right {
+		return -1
+	}
+
+	switch {
+	case target < arr[mid]:
+		right = mid
+	case target > arr[mid]:
+		left = mid
+	default:
+		return mid
+	}
+
+	return BinarySearchRecursive(arr, target, left, right)
+}
+
+// FindInsertPosition returns the index where the target should be inserted
+// to maintain the sorted order of the array.
+func FindInsertPosition(arr []int, target int) int {
+	left := 0
+	right := len(arr) - 1
+
+	if right == -1 {
+		return 0
+	}
+
+	if target < arr[left] {
+		return left
+	}
+
+	if target > arr[right] {
+		return right + 1
+	}
+
+	for left <= right {
+		mid := left + (right-left)/2
+
+		if mid == left || mid == right {
+			if arr[mid] < target {
+				return left + 1
+			} else {
+				return right - 1
+			}
+		}
+
+		switch {
+		case target < arr[mid]:
+			right = mid
+		case target > arr[mid]:
+			left = mid
+		default:
+			return mid
+		}
+
+	}
+
+	return 0
+
+}