Update

Author: ex01tus

README.md

@@ -2,7 +2,7 @@
 
 ![DSA Learning](https://img.shields.io/badge/DSA-Learning-blue?style=for-the-badge&logo=leetcode)
 
-This repository contains solutions for 526 DSA problems from the [LeetCode](https://leetcode.com/)
+This repository contains solutions for 530 DSA problems from the [LeetCode](https://leetcode.com/)
 website.
 
 Problems are separated into 19 topics:
@@ -20,13 +20,13 @@ Problems are separated into 19 topics:
 |  9 | Greedy Algorithms   | [greedy](src/greedy)                           |       21 |
 | 10 | Heap                | [heap](src/heap)                               |       19 |
 | 11 | Interval            | [interval](src/interval)                       |       11 |
-| 12 | Linked List         | [linked_list](src/linked_list)                 |       31 |
-| 13 | Math                | [math](src/math)                               |       19 |
+| 12 | Linked List         | [linked_list](src/linked_list)                 |       32 |
+| 13 | Math                | [math](src/math)                               |       20 |
 | 14 | Matrix              | [matrix](src/matrix)                           |       18 |
 | 15 | Quad Tree           | [quad_tree](src/quad_tree)                     |        1 |
 | 16 | Recursion           | [recursion](src/recursion)                     |       24 |
 | 17 | Stack               | [stack](src/stack)                             |       25 |
-| 18 | String              | [string](src/string)                           |       54 |
+| 18 | String              | [string](src/string)                           |       56 |
 | 19 | Trie                | [trie](src/trie)                               |        4 |
 
 ## Good places to start your own LeetCode journey

src/linked_list/DeleteNNodesAfterMNodesOfLinkedList.java

@@ -0,0 +1,39 @@
+package linked_list;
+
+/**
+ * Description: https://leetcode.com/problems/delete-n-nodes-after-m-nodes-of-a-linked-list
+ * Difficulty: Easy
+ * Time complexity: O(n)
+ * Space complexity: O(1)
+ */
+public class DeleteNNodesAfterMNodesOfLinkedList {
+
+    public ListNode deleteNodes(ListNode head, int m, int n) {
+        ListNode current = head;
+        ListNode prev = head;
+
+        int toKeep = m;
+        int toSkip = n;
+        while (current != null) {
+            if (toKeep > 0) {
+                prev = current;
+                current = current.next;
+                toKeep--;
+            } else if (toSkip > 0) {
+                prev.next = null;
+                current = current.next;
+                toSkip--;
+            } else {
+                prev.next = current;
+                toKeep = m;
+                toSkip = n;
+            }
+        }
+
+        return head;
+    }
+
+    private static class ListNode {
+        private ListNode next;
+    }
+}

src/math/MissingNumberInArithmeticProgression.java

@@ -0,0 +1,20 @@
+package math;
+
+/**
+ * Description: https://leetcode.com/problems/missing-number-in-arithmetic-progression
+ * Difficulty: Easy
+ * Time complexity: O(n)
+ * Space complexity: O(1)
+ */
+public class MissingNumberInArithmeticProgression {
+
+    public int missingNumber(int[] arr) {
+        int expectedSum = (int) ((arr[0] + arr[arr.length - 1]) / 2.0 * (arr.length + 1));
+        int actualSum = 0;
+        for (int num : arr) {
+            actualSum += num;
+        }
+
+        return expectedSum - actualSum;
+    }
+}

src/string/PalindromePermutation.java

@@ -0,0 +1,27 @@
+package string;
+
+/**
+ * Description: https://leetcode.com/problems/palindrome-permutation
+ * Difficulty: Easy
+ * Time complexity: O(n)
+ * Space complexity: O(1)
+ */
+public class PalindromePermutation {
+
+    public boolean canPermutePalindrome(String s) {
+        int[] freqMap = new int[26];
+        for (char c : s.toCharArray()) {
+            freqMap[c - 'a']++;
+        }
+
+        boolean isOddFound = false;
+        for (int freq : freqMap) {
+            if (freq % 2 != 0) {
+                if (isOddFound) return false;
+                isOddFound = true;
+            }
+        }
+
+        return true;
+    }
+}

src/string/SingleRowKeyboard.java

@@ -0,0 +1,27 @@
+package string;
+
+/**
+ * Description: https://leetcode.com/problems/single-row-keyboard
+ * Difficulty: Easy
+ * Time complexity: O(n)
+ * Space complexity: O(1)
+ */
+public class SingleRowKeyboard {
+
+    public int calculateTime(String keyboard, String word) {
+        int[] keyboardMap = new int[26];
+        for (int i = 0; i < keyboard.length(); i++) {
+            keyboardMap[keyboard.charAt(i) - 'a'] = i;
+        }
+
+        int prev = 0;
+        int time = 0;
+        for (int i = 0; i < word.length(); i++) {
+            int current = keyboardMap[word.charAt(i) - 'a'];
+            time += Math.abs(current - prev);
+            prev = current;
+        }
+
+        return time;
+    }
+}