add better solution for Combination Sum in README

Author: P-ppc

algorithms/CombinationSum/README.md

@@ -1,2 +1,26 @@
 # Combination Sum
-We can solve this proble by Backtracking Algorithm
+We can solve this proble by Backtracking Algorithm, the better solution is like below:
+```java
+class Solution {
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> res = new ArrayList<List<Integer>>();
+        backTrack(0, candidates, new ArrayList<Integer>(), target, res);
+        return res;
+    }
+    
+    public void backTrack(int beg, int[] candidates, List<Integer> temp, int target, List<List<Integer>> res) {
+        if (target == 0) {
+            res.add(temp);
+        } else {
+            for (int i = beg; i < candidates.length; i++) {
+                int c = candidates[i];
+                if (c <= target) {
+                    List<Integer> t = new ArrayList<Integer>(temp);
+                    t.add(c);
+                    backTrack(i, candidates, t, target - c, res);
+                }
+            }
+        }
+    }
+}
+```