Insertion Sort

Build the sorted prefix one item at a time, shifting larger values right until the current key can be inserted.

Algorithm

The checked-in replay follows the same small input and final output across all 21 DSA books, so this Go DSA implementation can be compared directly with the other languages.

Basic Implementation

basic.go

package main

import "fmt"

func main() {
	arr := []int{5, 1, 4, 2, 8}
	for i := 1; i < len(arr); i++ {
		key := arr[i]
		j := i - 1
		for j >= 0 && arr[j] > key {
			arr[j+1] = arr[j]
			j--
		}
		arr[j+1] = key
	}
	fmt.Println(arr)
}

Complexity

Time: O(n^2) worst and average, O(n) best
Space: O(1)
Stable: yes

Implementation notes

Go builds arr with the slice literal []int{5, 1, 4, 2, 8}; the slice header points at one backing array that is mutated in place.
The outer loop is for i := 1; i < len(arr); i++. Each pass copies arr[i] into local key before shifts overwrite that slot.
j := i - 1 is an int, so the loop can test j >= 0 before reading arr[j]; Go still performs normal bounds checks on each indexed access.
Shifts write arr[j+1] = arr[j], then arr[j+1] = key places the saved value after the loop opens the slot.
The trace records [5, 1, 4, 2, 8], then [1, 5, 4, 2, 8], [1, 4, 5, 2, 8], and [1, 2, 4, 5, 8]; the final 8 pass does not move.
fmt.Println(arr) uses Go's default slice formatting and prints [1 2 4 5 8], without commas.

sorted prefix Positions before the scan index are already sorted.

shifting Larger values move one slot right to make room for the key.