Add solution for Challenge 21 by rodney-b (#707)

* Add solution for Challenge 6 by rodney-b

* Add solution for Challenge 21 by rodney-b

* Implemented improvements suggested by CodeRabbit

* Refactored for consistency

---------

Co-authored-by: Rodney B <[email protected]>

Author: rodney-b

challenge-21/submissions/rodney-b/solution-template.go

@@ -0,0 +1,97 @@
+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 {
+	arrLen := len(arr)
+	left := 0
+	right := arrLen - 1
+
+	for left <= right {
+		mid := left + (right-left)/2 // avoids potential (left + right) overflow
+		// mid := int(uint(left+right) >> 1) // from Go's standard library
+		val := arr[mid]
+
+		switch {
+		case target < val:
+			right = mid - 1
+		case target > val:
+			left = mid + 1
+		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 left > right {
+		return -1
+	}
+
+	mid := left + (right-left)/2
+	val := arr[mid]
+
+	switch {
+	case target < val:
+		return BinarySearchRecursive(arr, target, left, mid-1)
+	case target > val:
+		return BinarySearchRecursive(arr, target, mid+1, right)
+	default:
+		return mid
+	}
+}
+
+// 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 {
+	arrLen := len(arr)
+	if arrLen == 0 {
+		return 0
+	}
+
+	left := 0
+	right := arrLen - 1
+
+	for left <= right {
+		mid := left + (right-left)/2
+		val := arr[mid]
+
+		switch {
+		case target < val:
+			right = mid - 1
+		case target > val:
+			left = mid + 1
+		default:
+			return mid // Insert before existing element
+		}
+	}
+
+	return left
+}