Chapter2-Selection-Sort.md

Chapter 2 - Selection Sort

Linked lists

• The item stores the address of the next item in the list.
• with a linked list, you never have to move your items
• The elements aren’t next to each other, so you can’t instantly calculate the position of the fifth element in memory.

Arrays

• Great if you want to read random elements. cuz you can look up any element in your array instantly.

	Arrays	Linked lists
Reading	O(1)	O(n)
inserting	O(n)	O(1) (if not count search)
Deletion	O(n)	O(1) (if not count search)

Selection sort

Suppose you have a bunch of music on your computer. For each artist, you have a play count. and you want to sort this list from most to least played.

1. Go through the list, find the most-played, and add it to the new list
2. Do it again to find the next-most-played, and add it to the new list
3. Do it until the list is empty.

To go throuh each item, take O(n). doing it n times. so the time complexity is O(n^2)

• selection sort is not very fast.

// time: O(n^2n => n^2)
function selectionSort(array) {
  const newArray = [];

  while(array.length) {
	  const smallestIndex = findSmallestIndex(array);
    newArray.push(array[smallestIndex]);
	  array.splice(smallestIndex, 1);     
  }
	return newArray;
}

function findSmallestIndex(array) {
  let min = array[0];
  let minIndex = 0;

  for(let i =0; i<array.length;i++) {
	  if(array[i]<min) {
		min = array[i];
        minIndex = i;
	  }
  }

	return minIndex;
}

• Recap
  • With array, all your elements are stored right next to each other
  • With a list, elements are strewn all over, and one element stores the address of the next one.
  • Arrays allow fast read
  • Linked lists allow fast insert and delete