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].

Basic Implementation

basic.ts 
const arr: number[] = [1, 2, 3, 4, 5, 6, 7];
let left: number = 0;
let right: number = arr.length - 1;
while (left < right) {
    const tmp: number = arr[left];
    arr[left] = arr[right];
    arr[right] = tmp;
    left = left + 1;
    right = right - 1;
}
console.log(JSON.stringify(arr));

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • TypeScript: use a temporary const tmp: number to swap two array slots. Never call arr.reverse(); the lesson is teaching the two-pointer walk.
  • Annotate left and right as : number so the index discipline is visible to the reader.
  • 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.
two pointers `left` starts at index `0`, `right` starts at `arr.length - 1`. Each loop iteration swaps `arr[left]` and `arr[right]` and moves the pointers toward each other.