invert_tree.js

function invert(xs) {
  let j = 1

  while(j < xs.length) {
    let start = j
    let end = 2*j

    while(end > start) {
      swap(xs, start, end)
      end--
      start++
    }

    j = 2*j+1
  }
}

function swap(xs, i, j) {
  let t = xs[i]
  xs[i] = xs[j]
  xs[j] = t
}

function print(arr, n) {
  n = n || 0
  if (n >= arr.length) { return }

  console.log(arr.slice(n, n*2+1).reduce((a,c) => a + " " + c))
  print(arr, n*2+1)
}

let xs = [4,2,7,1,3,6,9]

print(xs)
invert(xs)
print(xs)