1 00:00:00,420 --> 00:00:07,920 ‫So now we're going to write our function for delete last, and deleting the last item in a doubly linked 2 00:00:07,920 --> 00:00:12,680 ‫list is very different than doing it in a singly linked list. 3 00:00:12,690 --> 00:00:20,850 ‫And the reason why is moving tail to the left in a singly linked list was o of n because we had to start 4 00:00:20,850 --> 00:00:27,180 ‫at the head and iterate through the linked list to get to this node and point tail to it. 5 00:00:27,300 --> 00:00:34,560 ‫But with a doubly linked list, we have arrows going the other way and that is going to simplify things. 6 00:00:34,770 --> 00:00:41,640 ‫So in order to do this we're going to have a variable temp that points at that tail node and to move 7 00:00:41,670 --> 00:00:50,280 ‫tail over, we'll just set it equal to tail previous and that moves that over as an O of one operation 8 00:00:50,280 --> 00:00:55,650 ‫instead of it being o of n like it was with a singly linked list. 9 00:00:55,890 --> 00:01:05,730 ‫And then we'll set tail next to be equal to null pointer that does this and then we'll say delete temp 10 00:01:05,730 --> 00:01:09,000 ‫and that will delete that node from memory. 11 00:01:09,330 --> 00:01:16,140 ‫And just like we had with a singly linked list, we have an edge case for when we have one node and 12 00:01:16,140 --> 00:01:17,610 ‫our doubly linked list. 13 00:01:17,700 --> 00:01:22,920 ‫And we also have an edge case for when we have an empty, doubly linked list. 14 00:01:23,250 --> 00:01:25,890 ‫So we'll start our code out like this. 15 00:01:26,100 --> 00:01:29,550 ‫And the first thing we'll code for is this situation here. 16 00:01:29,550 --> 00:01:36,450 ‫When the linked list is empty, we'll say if the length is equal to zero, we'll just run return. 17 00:01:37,150 --> 00:01:41,620 ‫So now let's code for when we have one item in the linked list. 18 00:01:41,890 --> 00:01:46,440 ‫The first thing we'll do is create that variable temp and point it to tail. 19 00:01:46,450 --> 00:01:54,190 ‫We'll do it like this temp equals tail, and then we'll say if the length is equal to one set, head 20 00:01:54,190 --> 00:01:56,980 ‫and tail to be equal to null pointer. 21 00:01:57,580 --> 00:01:58,660 ‫Like this. 22 00:01:59,020 --> 00:02:02,060 ‫So now let's add this in with the rest of our code. 23 00:02:02,080 --> 00:02:08,110 ‫So we've written a code for when the length list is empty and for when we have one item. 24 00:02:08,320 --> 00:02:11,680 ‫And now for two or more items we'll say else. 25 00:02:11,680 --> 00:02:14,830 ‫And now let's build out this else statement. 26 00:02:15,160 --> 00:02:19,870 ‫So we're going to set tail to be equal to tail previous. 27 00:02:20,320 --> 00:02:22,040 ‫We'll do that with this line of code. 28 00:02:22,060 --> 00:02:25,840 ‫Tail equals tail previous, and that moves that over. 29 00:02:26,230 --> 00:02:31,120 ‫Then we'll set tail next to be equal to null pointer. 30 00:02:31,420 --> 00:02:33,610 ‫And that does that. 31 00:02:33,610 --> 00:02:38,020 ‫And we'll do that with this line of code tail next equals null pointer. 32 00:02:38,290 --> 00:02:43,960 ‫So let's add this in with the rest of our code and then we just have a couple more steps. 33 00:02:43,960 --> 00:02:52,120 ‫First, we'll say delete temp and that will remove that node from memory and then we'll decrement the 34 00:02:52,120 --> 00:02:52,840 ‫length. 35 00:02:53,200 --> 00:03:00,730 ‫So now let's add these and with the rest of our code, and that is the entire delete last function. 36 00:03:01,180 --> 00:03:04,510 ‫So we'll look at this code in a moment in V's code. 37 00:03:04,510 --> 00:03:12,820 ‫And when we do, we'll create this doubly linked list and we'll run delete last three different times 38 00:03:12,820 --> 00:03:19,750 ‫to cover all three of the situations we coded for for when we have two or more items in the length list 39 00:03:19,750 --> 00:03:22,750 ‫and when we have one item in the length list. 40 00:03:22,750 --> 00:03:27,640 ‫And then we'll run it again to make sure it works when we have an empty linked list. 41 00:03:28,000 --> 00:03:31,360 ‫So now let's flip over and take a look at this. 42 00:03:31,900 --> 00:03:34,660 ‫So there is our delete last member function. 43 00:03:34,660 --> 00:03:39,970 ‫They're added to our doubly linked list class and I'll scroll up. 44 00:03:41,090 --> 00:03:47,720 ‫And in our main function, this creates that doubly linked list with two nodes with the values of one 45 00:03:47,720 --> 00:03:48,710 ‫and two. 46 00:03:48,740 --> 00:03:50,630 ‫With this will print that out. 47 00:03:50,630 --> 00:03:55,610 ‫This will be before we have run delete last and I'll scroll up again. 48 00:03:56,120 --> 00:03:59,690 ‫And with this we'll run delete last three different times. 49 00:03:59,870 --> 00:04:06,050 ‫The first time to make sure it works when we have two items and then with one item and then we'll do 50 00:04:06,050 --> 00:04:08,990 ‫it again when the linked list is empty. 51 00:04:09,410 --> 00:04:12,860 ‫So I'll run this and this is our linked list. 52 00:04:12,860 --> 00:04:14,750 ‫Before we ran to last. 53 00:04:14,750 --> 00:04:20,840 ‫It has items with one and two and this is what it looks like after we deleted the last item and then 54 00:04:20,840 --> 00:04:21,920 ‫we ran it again here. 55 00:04:21,920 --> 00:04:27,860 ‫That created an empty linked list and that we ran it a third time to make sure it works when the linked 56 00:04:27,860 --> 00:04:29,420 ‫list is empty. 57 00:04:30,240 --> 00:04:34,950 ‫And that is our function for delete last.