Time Complexity = O(n), Space Complexity = O(1)

reverseArr.java

public static void reverseArr(int arr[]){
  int n = arr.length;
  for(int i=0; i<n/2;i++){
    int temp = arr[i];
    arr[i] = arr[n-i-1];
    arr[n-i-1] = temp;
  }
}