pub fn plus_one(digits: Vec<i32>) -> Vec<i32> {
  // Keep track of when to carry for the next digit.
  let mut carry = false;

  // Reverse the digits so they're from least to most significant.
  let mut digits = digits.into_iter().rev();

  // Create a vector to store the incremented digits.
  let mut incremented = vec![];

  while let Some(digit) = digits.next() {
    if digit == 9 {
      // When the current digit is 9 save 0 and keep going through the digits.
      incremented.push(0);
      carry = true;
    } else {
      // When the digit isn't 9, increment it and save it, set carry to false
      // and break the loop early.
      incremented.push(digit + 1);
      carry = false;
      break;
    }
  }

  // If the last digit was a 9 we have to add an extra 1, for example:
  // [9] turns into [0] and then here we make it [0, 1].
  if carry {
    incremented.push(1);
  }

  // Then reverse both iterators so they're in the correct order and chain them.
  digits.rev().chain(incremented.into_iter().rev()).collect()
}

#[test]
fn test_plus_one() {
  assert_eq!(plus_one(vec![1, 2, 3]), vec![1, 2, 4]);
  assert_eq!(plus_one(vec![4, 3, 2, 5]), vec![4, 3, 2, 6]);
  assert_eq!(plus_one(vec![9]), vec![1, 0]);
  assert_eq!(plus_one(vec![9, 9]), vec![1, 0, 0]);
  assert_eq!(plus_one(vec![7, 9, 8]), vec![7, 9, 9]);
  assert_eq!(plus_one(vec![8, 9, 9]), vec![9, 0, 0]);
}