Add solution for Challenge 21 (#840)

Co-authored-by: go-interview-practice-bot[bot] <230190823+go-interview-practice-bot[bot]@users.noreply.github.com>

Author: duplabe

challenge-21/submissions/duplabe/solution-template.go

@@ -0,0 +1,75 @@
+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)
+}
+
+// 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, right := 0, len(arr)-1
+	for left <= right {
+		mid := (left + right) / 2
+		if arr[mid] < target {
+			left = mid + 1
+		} else if arr[mid] > target {
+			right = mid - 1
+		} else {
+			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 left > right {
+		return -1
+	}
+
+	mid := (left + right) / 2
+	if arr[mid] == target {
+		return mid
+	} else if arr[mid] < target {
+		return BinarySearchRecursive(arr, target, mid+1, right)
+	} else {
+		return BinarySearchRecursive(arr, target, left, mid-1)
+	}
+}
+
+// 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, right := 0, len(arr)
+	for left < right {
+		mid := (left + right) / 2
+		if arr[mid] < target {
+			left = mid + 1
+		} else {
+			right = mid
+		}
+	}
+
+	return left
+}