update 31

Author: fishercoder1534

Diff for: src/main/java/com/fishercoder/solutions/_31.java

@@ -8,7 +8,12 @@ public static class Solution1 {
          * 1. if the array is already in decrementing order, then there's no next larger permutation possible.
          * 2. if not, start from the end of the array, find the first pair of numbers that break the decrementing order
          * 3. then from that index going to the right again, find the element that is closest bigger than this number, swap them
-         * 4. reverse the right half of this array after this index
+         * 4. reverse the right half of this array after this index to make it sorted increasingly so that it's the next permutation
+         * <p>
+         * Use this nums as an example: 1,2,5,4,3
+         * 1. we first found such an adjacent pair: 2 and 5 that breaks the decrementing order;
+         * 2. then starting from the right side again, we find a number that's bigger than 2, so that is 3, swap them, the array becomes: 1,3,5,4,2
+         * 3. reverse this sub-array: 5,4,2 since they are guaranteed to be decreasing order, so reversing them will give us the next permutation: 1,3,2,4,5
          */
 
         public void nextPermutation(int[] nums) {
@@ -31,9 +36,7 @@ public void nextPermutation(int[] nums) {
         private void reverse(int[] nums, int start) {
             int end = nums.length - 1;
             while (start <= end) {
-                int tmp = nums[start];
-                nums[start++] = nums[end];
-                nums[end--] = tmp;
+                swap(nums, start++, end--);
             }
         }
 

Diff for: src/test/java/com/fishercoder/_31Test.java

@@ -35,4 +35,18 @@ public void test3() {
         solution1.nextPermutation(nums);
         assertArrayEquals(new int[]{1, 2, 6, 1, 2, 3, 4}, nums);
     }
+
+    @Test
+    public void test4() {
+        nums = new int[]{1, 2, 5, 4, 3};
+        solution1.nextPermutation(nums);
+        assertArrayEquals(new int[]{1, 3, 2, 4, 5}, nums);
+    }
+
+    @Test
+    public void test5() {
+        nums = new int[]{3, 2, 1};
+        solution1.nextPermutation(nums);
+        assertArrayEquals(new int[]{1, 2, 3}, nums);
+    }
 }