Combination Sum III

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题目：

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

分析：

求子集问题类似！

参考代码如下：

public class Solution {    public List<List<Integer>> combinationSum3(int k, int n) {        List<List<Integer>> result = new ArrayList<List<Integer>>();        List<Integer> list = new ArrayList<Integer>();        dfs(result, 1, n, list, k);        return result;    }         public void dfs(List<List<Integer>> result, int start, int sum, List<Integer> list, int k){        if(sum==0 && list.size()==k){            List<Integer> temp = new ArrayList<Integer>();            temp.addAll(list);            result.add(temp);        }             for(int i=start; i<=9; i++){            if(sum-i<0) break;            if(list.size()>k) break;                 list.add(i);            dfs(result, i+1, sum-i, list, k);            list.remove(list.size()-1);        }    }}

public class Solution {    public List<List<Integer>> combinationSum3(int k, int n) {        List<List<Integer>> result = new ArrayList<List<Integer>>();        List<Integer> list = new ArrayList<Integer>();        dfs(result, 1, n, list, k);        return result;    }         public void dfs(List<List<Integer>> result, int start, int sum, List<Integer> list, int k){        if(sum==0 && list.size()==k){            List<Integer> temp = new ArrayList<Integer>();            temp.addAll(list);            result.add(temp);        }             for(int i=start; i<=9; i++){            if(sum-i<0) break;            if(list.size()>k) break;                 list.add(i);            dfs(result, i+1, sum-i, list, k);            list.remove(list.size()-1);        }    }}

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