# How to Sort linked list of 0s, 1s and 2s

[Question]: Given a singly linked list head sort it with with all with zeros, ones and two’s
Example:
Input: 1 -> 1 -> 2 -> 0 -> 2 -> 0 -> 1 -> NULL
Output: 0 -> 0 -> 1 -> 1 -> 1 -> 2 -> 2 -> NULL

Approach: Create count array which stores count of 0’s, 1’s and 2’s occurrences. We will iterate over linked list & finds the count of each(0,1,2)
Create a new head named `current` and replace items with sorted order

``````// TC: O(n)
// SC: O(n)
func sortList012(_ head: ListNode?) {
var count = [ 0, 0, 0 ]
// Count the number of 0's, 1's, and 2's in the
// linked list
var current = head
while current != nil {
count[current!.val]+=1// Filling respective index like count of 0's 1's & 2's
current = current?.next
}
// Overwrite the linked list with the sorted
// elements
current = head
var i = 0
while current != nil {
if count[i] == 0 {
i+=1
} else {
current?.val = i
count[i]-=1
current = current?.next
}
}
}

let zeroOneTwoLinkedList = ListNode(1, ListNode(2, ListNode(0,ListNode(2, ListNode(0)))))
sortList012(zeroOneTwoLinkedList)
//print("sorted by 0,1,2", zeroOneTwoLinkedList)// 0 0 1 2 1``````