From a030b3a8d1ced03be98512b13acb7eb9ca74ce3f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=94=90=E6=A0=91=E6=A3=AE?= <tangshusen@pku.edu.cn>
Date: Tue, 10 Sep 2019 20:01:43 +0800
Subject: [PATCH] Create 216. Combination Sum III.md

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+# [216. Combination Sum III](https://leetcode.com/problems/combination-sum-iii/)
+
+# 思路
+从1-9十个数字中选出k个数字和为n, 要求返回所有组合. 
+
+这种求所有组合的题一般就是DFS, 此题为了避免重复, 我们需要一个start标记, 每次从start开始进行递归DFS调用而不是每次都从1. 
+另外, 由于有了start标记, 我们也不需要使用visited数组了.
+
+# C++
+``` C++
+class Solution {
+private:
+    void DFS(int start, int k, int n, vector<vector<int>>&res, vector<int>&out){
+        if(k < 0 || n < 0) return;
+        if(k == 0 || n == 0){
+            if(n == k) res.push_back(out);
+            return;
+        }
+        // 每次从start开始进行递归调用而不是每次都从1
+        for(int i = start; i <= 9; i++){
+            out.push_back(i);
+            DFS(i + 1, k - 1, n - i, res, out);
+            out.pop_back();
+        }
+    }
+    
+public:
+    vector<vector<int>> combinationSum3(int k, int n) {
+        vector<vector<int>>res;
+        vector<int>out;
+        
+        DFS(1, k, n, res, out);
+        return res;
+    }
+};
+```