Added task 31.

Author: javadev

src/main/java/s0031.next.permutation/Solution.java

@@ -0,0 +1,27 @@
+package s0031.next.permutation;
+
+public class Solution {
+    public void nextPermutation(int[] nums) {
+        if (nums == null || nums.length <= 1) return;
+
+        int i = nums.length - 2;
+        while (i >= 0 && nums[i] >= nums[i + 1]) i--;
+
+        if (i >= 0) {
+            int j = nums.length - 1;
+            while (nums[j] <= nums[i]) j--;
+            swap(nums, i, j);
+        }
+        reverse(nums, i + 1, nums.length - 1);
+    }
+
+    public void swap(int[] nums, int i, int j) {
+        int temp = nums[i];
+        nums[i] = nums[j];
+        nums[j] = temp;
+    }
+
+    public void reverse(int[] nums, int i, int j) {
+        while (i < j) swap(nums, i++, j--);
+    }
+}

src/test/java/s0031.next.permutation/SolutionTest.java

@@ -0,0 +1,15 @@
+package s0031.next.permutation;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.Test;
+
+public class SolutionTest {
+    @Test
+    public void nextPermutation() {
+        int[] array = new int[] {1, 2, 3};
+        new Solution().nextPermutation(array);
+        assertThat(java.util.Arrays.toString(array), equalTo("[1, 3, 2]"));
+    }
+}