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Author: Chris Wu

README.md

@@ -1,5 +1,5 @@
 # Leetcode Python Solution
-1. This is my Python solution on Leetcode. The question is at `problems/the-file-name/`. For example, `merge-sorted-array.py`'s question is at `https://leetcode.com/problems/merge-sorted-array/`.
+1. This is my Python (2.7) solution on Leetcode. The question is at `problems/the-file-name/`. For example, `merge-sorted-array.py`'s question is at `https://leetcode.com/problems/merge-sorted-array/`.
 
 2. I really take time tried to make the best solution or explaination. 
 Because I wanted to help others like me. 

problems/combination-sum-iii.py

@@ -0,0 +1,27 @@
+"""
+If you haven't seen [this problem](https://leetcode.com/problems/combinations/), I suggest you do that first!
+And here is the [explaination](https://leetcode.com/problems/combinations/discuss/387753) for that problem.
+
+This problem is basically the same as [that problem](https://leetcode.com/problems/combinations/) with its N=9.
+The difference is we have to find the combination that sum up as N.
+So all we have to do is change the condition of [0]. And the condition for backtracking [1].
+"""
+class Solution(object):
+    def combinationSum3(self, K, N):
+        answer = []
+        ans = []
+        first = 1
+        total = 0
+        while True:
+            if len(ans)==K and total==N: #[0]
+                answer.append(ans[:])
+
+            if len(ans)==K or total>N or first>9: #[1]
+                if not ans: return answer
+                first = ans.pop() #backtrack
+                total-=first
+                first+=1
+            else:
+                ans.append(first)
+                total+=first
+                first+=1