# Combination Sum

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:

• All numbers (including target) will be positive integers.
• Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
• The solution set must not contain duplicate combinations.

For example, given candidate set 2,3,6,7 and target 7,

A solution set is:

``````[7]
[2, 2, 3]
``````

## Solution

Sort & Recursion 经典的 dfs 回溯法

## Code

``````public class Solution {
public List<List<Integer>> combinationSum(int[] candidates, int target) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
Arrays.sort(candidates);
ArrayList<Integer> path = new ArrayList<Integer>();
combinationSumRe(candidates, target, 0, path, res);
return res;
}

void combinationSumRe(int[] candidates, int target, int start, ArrayList<Integer> path, List<List<Integer>> res) {
if (target == 0) {
ArrayList<Integer> p = new ArrayList<Integer>(path);
res.add(p);
return;
}
for (int i = start;
i < candidates.length && target >= candidates[i]; ++i) {
path.add(candidates[i]);
combinationSumRe(candidates, target-candidates[i], i, path, res);
path.remove(path.size() - 1);
}
}
}
``````