Arrays and Iteration

Reverse Array In Place (Two Pointers)

Walk two indices toward each other from the ends of the array, swapping at each step. Stops when the indices meet or cross. Demonstrates the two-pointer pattern with the smallest possible state.

Algorithm

Canonical input [1, 2, 3, 4, 5, 6, 7] (odd length, middle element stays put) yields three swap frames and reverses to [7, 6, 5, 4, 3, 2, 1].

two pointers `left` starts at index `0`, `right` starts at `n - 1`. Each loop iteration swaps `arr[left]` and `arr[right]` and moves the pointers toward each other.

Basic Implementation

basic.go
Replay: real traced execution (multi-file project)
package main

import "fmt"

func main() {
	arr := []int{1, 2, 3, 4, 5, 6, 7}
	left := 0
	right := len(arr) - 1
	for left < right {
		arr[left], arr[right] = arr[right], arr[left]
		left = left + 1
		right = right - 1
	}
	fmt.Println(arr)
}

  1. arr ← [1, 2, 3, 4, 5, 6, 7]

    5func main() {6	arr := []int{1, 2, 3, 4, 5, 6, 7}7	left := 0
    values this step[1, 2, 3, 4, 5, 6, 7]arr

  2. left ← 0

    6arr := []int{1, 2, 3, 4, 5, 6, 7}7left := 08right := len(arr) - 1
    values this step0left[1, 2, 3, 4, 5, 6, 7]arr

  3. right ← 6

    7left := 08right := len(arr) - 19for left < right {
    values this step6right0left

  4. arr ← [7, 2, 3, 4, 5, 6, 1]

    9for left < right {10	arr[left], arr[right] = arr[right], arr[left]11	left = left + 1
    values this step[1, 2, 3, 4, 5, 6, 7] → [7, 2, 3, 4, 5, 6, 1]arr0left6right

  5. left ← 1

    10arr[left], arr[right] = arr[right], arr[left]11left = left + 112right = right - 1
    values this step0 → 1left

  6. right ← 5

    11	left = left + 112	right = right - 113}
    values this step6 → 5right

  7. arr ← [7, 6, 3, 4, 5, 2, 1]

    9for left < right {10	arr[left], arr[right] = arr[right], arr[left]11	left = left + 1
    values this step[7, 2, 3, 4, 5, 6, 1] → [7, 6, 3, 4, 5, 2, 1]arr1left5right

  8. left ← 2

    10arr[left], arr[right] = arr[right], arr[left]11left = left + 112right = right - 1
    values this step1 → 2left

  9. right ← 4

    11	left = left + 112	right = right - 113}
    values this step5 → 4right

  10. arr ← [7, 6, 5, 4, 3, 2, 1]

    9for left < right {10	arr[left], arr[right] = arr[right], arr[left]11	left = left + 1
    values this step[7, 6, 3, 4, 5, 2, 1] → [7, 6, 5, 4, 3, 2, 1]arr2left4right

  11. left ← 3

    10arr[left], arr[right] = arr[right], arr[left]11left = left + 112right = right - 1
    values this step2 → 3left

  12. right ← 3

    11	left = left + 112	right = right - 113}
    values this step4 → 3right

  13. for left < right

    8right := len(arr) - 19for left < right {10	arr[left], arr[right] = arr[right], arr[left]
    values this step[7, 6, 5, 4, 3, 2, 1]arr3left3right

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Go: idiomatic tuple swap arr[left], arr[right] = arr[right], arr[left] keeps the swap step visible without leaning on a helper that hides the lesson; the standard library has no in-place reverse for []int.
  • left := 0 and right := len(arr) - 1 are plain int indices; the left < right guard handles the meet-in-the-middle exit honestly for the odd-length canonical input.
  • The replay shows both left and right, the values about to be swapped, and the array contents after the swap. The loop-exit frame is the moment the pointers meet.