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function reverseWords(message) {
  // O(n)
  // first reverse all chars in the message
  reverse(message, 0, message.length - 1);

  // O(n)
  // then re-reverse each word
  let left = 0;
  for (let i = 0; i <= message.length; i++) {
    if (message[i] === ' ' || i === message.length) {
      reverse(message, left, i - 1);
      left = i + 1;
    }
  }
}

function reverse(message, left, right) {
  // O(n)
  while (left < right) {
    const temp = message[left];
    message[left] = message[right];
    message[right] = temp;
    left++;
    right--;
  }
}

// Tests

let desc = 'one word';
let input = 'vault'.split('');
reverseWords(input);
let actual = input.join('');
let expected = 'vault';
assertEqual(actual, expected, desc);

desc = 'two words';
input = 'thief cake'.split('');
reverseWords(input);
actual = input.join('');
expected = 'cake thief';
assertEqual(actual, expected, desc);

desc = 'three words';
input = 'one another get'.split('');
reverseWords(input);
actual = input.join('');
expected = 'get another one';
assertEqual(actual, expected, desc);

desc = 'multiple words same length';
input = 'rat the ate cat the'.split('');
reverseWords(input);
actual = input.join('');
expected = 'the cat ate the rat';
assertEqual(actual, expected, desc);

desc = 'multiple words different lengths';
input = 'yummy is cake bundt chocolate'.split('');
reverseWords(input);
actual = input.join('');
expected = 'chocolate bundt cake is yummy';
assertEqual(actual, expected, desc);

desc = 'empty string';
input = ''.split('');
reverseWords(input);
actual = input.join('');
expected = '';
assertEqual(actual, expected, desc);

function assertEqual(a, b, desc) {
  if (a === b) {
    console.log(`${desc} ... PASS`);
  } else {
    console.log(`${desc} ... FAIL: ${a} != ${b}`);
  }
}