From b6f6c626e70f694de4108431eba08fad04126a16 Mon Sep 17 00:00:00 2001
From: liukw-go <1817148780@qq.com>
Date: Thu, 7 Aug 2025 19:42:21 +0800
Subject: [PATCH] Add solution for Challenge 21

---
 .../submissions/liukw-go/solution-template.go | 81 +++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 challenge-21/submissions/liukw-go/solution-template.go

diff --git a/challenge-21/submissions/liukw-go/solution-template.go b/challenge-21/submissions/liukw-go/solution-template.go
new file mode 100644
index 00000000..d71dc276
--- /dev/null
+++ b/challenge-21/submissions/liukw-go/solution-template.go
@@ -0,0 +1,81 @@
+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 {
+	// TODO: Implement this function
+	n, ret := len(arr), -1
+	l, r := 0, n - 1
+	for l <= r {
+	    m := (l + r) >> 1
+	    if arr[m] >= target {
+	        r = m - 1
+	        ret = m
+	    } else {
+	        l = m + 1
+	    }
+	}
+	if ret == -1 { return -1 }
+	if target != arr[ret] { return -1 }
+	return ret
+}
+
+// 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 {
+	// TODO: Implement this function
+	l, r, ret := left, right, -1
+	for l <= r {
+	    m := (l + r) >> 1
+	    if arr[m] >= target {
+	        r = m - 1
+	        ret = m
+	    } else {
+	        l = m + 1
+	    }
+	}
+	if ret == -1 { return -1 }
+	if target != arr[ret] { return -1 }
+	return ret
+}
+
+// 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 {
+	// TODO: Implement this function
+	n, ret := len(arr), len(arr)
+	l, r := 0, n - 1
+	for l <= r {
+	    m := (l + r) >> 1
+	    if arr[m] >= target {
+	        r = m - 1
+	        ret = m
+	    } else {
+	        l = m + 1
+	    }
+	}
+	return ret
+}