Given an array of integers, find the first missing positive integer in linear time and constant space

Question

In other words, find the lowest positive integer that does not exist in the array. The array can contain duplicates and negative numbers as well. This question was asked by Stri

Answer 1

Javascript implementation of PMCarpan's algorithm

function getMissingInt(array) {
  const segArray = array.filter((a) => a > 0);
  for (let i = 0; i < segArray.length; i++) {
    const value = Math.abs(segArray[i]);
    if (value <= segArray.length && segArray[value - 1] > 0) {
      segArray[value - 1] *= -1;
    }
  }
  for (let i = 0; i < segArray.length; i++) {
    if (segArray[i] > 0) {
      return i + 1;
    }
  }
  return segArray.length + 1;
}
console.log(getMissingInt([1, -1, -5, -3, 3, 4, 2, 8]));
console.log(getMissingInt([3, 4, -1, 1]));
console.log(getMissingInt([1, 2, 0]));