From bf1bfdf47333c9434353671c819c9d113a7e9760 Mon Sep 17 00:00:00 2001
From: "go-interview-practice-bot[bot]"
 <230190823+go-interview-practice-bot[bot]@users.noreply.github.com>
Date: Wed, 19 Nov 2025 10:28:42 +0000
Subject: [PATCH] Add solution for Challenge 21

---
 .../submissions/yz4230/solution-template.go   | 78 +++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 challenge-21/submissions/yz4230/solution-template.go

diff --git a/challenge-21/submissions/yz4230/solution-template.go b/challenge-21/submissions/yz4230/solution-template.go
new file mode 100644
index 00000000..a1c04a1a
--- /dev/null
+++ b/challenge-21/submissions/yz4230/solution-template.go
@@ -0,0 +1,78 @@
+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)
+	for left < right {
+	    mid := (right - left) / 2 + left
+	    midv := arr[mid]
+	    if target < midv {
+	        right = mid
+	    } else if midv < target {
+	        left = 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 := (right - left) / 2 + left
+	midv := arr[mid]
+	if target < midv {
+	    return BinarySearchRecursive(arr, target, left, mid-1)
+	} else if midv < target {
+	    return BinarySearchRecursive(arr, target, mid+1, right)
+	} else {
+	    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 {
+	left, right := 0, len(arr)
+	for left < right {
+	    mid := (right - left) / 2 + left
+	    midv := arr[mid]
+	    if target < midv {
+	        right = mid
+	    } else if midv < target {
+	        left = mid + 1
+	    } else {
+	        return mid
+	    }
+	}
+	return left
+}