packageeasy;

importclasses.ListNode;

publicclassSwapNodesinPairs {

publicListNodeswapPairs(ListNodehead) {

if(head ==null ||head.next ==null)returnhead;

ListNodesecond =head.next;

ListNodethird =second.next;

second.next =head;

head.next =swapPairs(third);

returnsecond;

}

}

|58|[Length of Last Word](https://leetcode.com/problems/length-of-last-word/)|[Solution](../../blob/master/EASY/src/easy/LengthofLastWord.java)|O(n)|O(1)|Easy|

|56|[Merge Intervals](https://leetcode.com/problems/merge-intervals/)|[Solution](../../blob/master/HARD/src/hard/MergeIntervals.java)|O(n*logn)|O(1)|Hard|

|43|[Multiply Strings](https://leetcode.com/problems/multiply-strings/)|[Solution]|||Medium

|24|[Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/)|[Solution](../../blob/master/EASY/src/easy/SwapNodesinPairs.java)|O(n)|O(1)|Easy| Recursion, LinkedList

|23|[Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)|[Solution](../../blob/master/HARD/src/hard/MergeKSortedList.java)|O(n*logk)|O(logk)|Hard|Heap

|21|[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)|[Solution](../../blob/master/EASY/src/easy/MergeTwoSortedLists.java)|O(n)|O(1)|Easy|

|17|[Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)|[Solution](../../blob/master/MEDIUM/src/medium/LetterCombinationsofaPhoneNumber.java)|O(n*4^n)|O(n)|Medium|Backtracking