add combination sum

Author: tg123

combination-sum/README.md

@@ -1,4 +1,12 @@
+## Apply DFS pattern
 
-## TODO 
-  * write down thinking
+See DFS pattern in [N-Queens](../n-queens)
 
+
+  * `steps`: choose a candidate number
+  * `choices each step`: 0..INFINITE times candidate number
+  * `possibility checker`: sum of current numbers is less than or equals to target number
+
+### Backtracking
+
+Remove the selected candidate number from steps.