add restore ip addr

restore-ip-addresses/README.md

@@ -1,4 +1,14 @@
+## Apply DFS pattern
 
-## TODO 
- * write down thinking
+See DFS pattern in [N-Queens](../n-queens)
 
+
+ * `steps`: 4 part of an ip-address
+ * `choices each step`: 1-3 chars from the string
+ * `possibility checker`: the number formed by chars should be less than or equals to 255
+
+### When to use backtracking
+
+In this case, I do not use `stack[pstack] = null` after starting to try another choice.
+
+Because, the value in `stack[pstack]` will not affect any further choice. Rollbacking is not needed and the value will be overwrited.