add permutations ii

Author: tg123

permutations-ii/README.md

@@ -1,4 +1,23 @@
+## Variant of [Next Permutation](../next-permutation)
 
-## TODO 
-  * write down thinking
+With the help of [Next Permutation](../next-permutation), you can find all unique permutations from the first permutation.
 
+### First and Last
+
+ * First permutation is the numbers in non-descending order
+ * Last permutation is the numbers in non-ascending order
+
+
+so the code is like
+
+```
+sort(num) // make it the first
+
+
+while num is not the last
+  call nextPermutation(num)
+  
+  copy num into result
+
+
+```