Add solution for Challenge 21 (#670)

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

Author: kiramux

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

@@ -0,0 +1,80 @@
+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 {
+// 左闭右开
+left := 0
+right := len(arr)
+for left < right {
+mid := left + (right-left)>>1
+if target == arr[mid] {
+return mid
+} else if target < arr[mid] {
+right = mid
+} else {
+left = mid + 1
+}
+}
+
+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 {
+// test 中是左闭右闭，因此按左闭右闭写
+if left > right {
+return -1
+}
+mid := left + (right-left)>>1
+if target == arr[mid] {
+return mid
+} else if target < arr[mid] {
+return BinarySearchRecursive(arr, target, left, mid-1)
+} else {
+return BinarySearchRecursive(arr, target, mid+1, 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 {
+// 左闭右开，终止时 left 和 right 等价
+left := 0
+right := len(arr)
+for left < right {
+mid := left + (right-left)>>1
+if target == arr[mid] {
+right = mid
+} else if target < arr[mid] {
+right = mid
+} else {
+left = mid + 1
+}
+}
+return left
+}