solves nextpermutation

Author: anishLearnsToCode

Diff for: README.md

@@ -36,6 +36,7 @@
 |  27  | [Remove Element](https://leetcode.com/problems/remove-element/)                                                                                           |                                     [![Java](assets/java.png)](src/RemoveElement.java) [![Python](assets/python.png)](python/remove_element.py)                                      |                                                                                                                                   |
 |  28  | [Needle in Haystack](https://leetcode.com/problems/implement-strstr)                                                                                      |                                  [![Java](assets/java.png)](src/NeedleInHaystack.java) [![Python](assets/python.png)](python/needle_in_haystack.py)                                  |                                                                                                                                   |
 |  29  | [Divide Two Integers](https://leetcode.com/problems/divide-two-integers)                                                                                  |                                                                [![Java](assets/java.png)](src/DivideTwoIntegers.java)                                                                |                                                                                                                                   |
+|  31  | [Next Permutation](https://leetcode.com/problems/next-permutation)                                                                                        |                                                                 [![Java](assets/java.png)](src/NextPermutation.java)                                                                 |                                                                                                                                   |
 |  35  | [Search Inserted Position](https://leetcode.com/problems/search-insert-position/)                                                                         |                              [![Java](assets/java.png)](src/SearchInsertPosition.java) [![Python](assets/python.png)](python/search_insert_position.py)                              |                                                                                                                                   |
 |  38  | [Count and Say](https://leetcode.com/problems/count-and-say)                                                                                              |                                       [![Java](assets/java.png)](src/CountAndSay.java) [![Python](assets/python.png)](python/count_and_say.py)                                       |                                                                                                                                   |
 |  53  | [Maximum SubArray](https://leetcode.com/problems/maximum-subarray)                                                                                        |                                 [![Java](assets/java.png)](src/MaximumSubArray.java) [![Python](assets/python.png)](python/maximum_sum_subarray.py)                                  |                                                                                                                                   |

Diff for: src/NextPermutation.java

@@ -0,0 +1,42 @@
+// https://leetcode.com/problems/next-permutation
+// T: O(|nums|)
+// S: O(1)
+
+public class NextPermutation {
+    public void nextPermutation(int[] nums) {
+        for (int i = nums.length - 2 ; i >= 0 ; i--) {
+            if (nums[i] < nums[i + 1]) {
+                int index = indexOfNextLargest(nums, i + 1, nums[i]);
+                swap(nums, i, index);
+                reverse(nums, i + 1, nums.length);
+                return;
+            }
+        }
+        reverse(nums);
+    }
+
+    private void swap(int[] array, int i, int j) {
+        int temp = array[i];
+        array[i] = array[j];
+        array[j] = temp;
+    }
+
+    private int indexOfNextLargest(int[] array, int start, int x) {
+        for (int i = start ; i < array.length ; i++) {
+            if (array[i] <= x) {
+                return i - 1;
+            }
+        }
+        return array.length - 1;
+    }
+
+    private void reverse(int[] array) {
+        reverse(array, 0, array.length);
+    }
+
+    private void reverse(int[] array, int start, int end) {
+        for (int i = start ; i < start + (end - start) / 2 ; i++) {
+            swap(array, i, end - i + start - 1);
+        }
+    }
+}