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import java.util.LinkedList;

import java.util.List;

public class CombinationSum {

// 耗时23ms

public List<List<Integer>> combinationSum(int[] candidates, int target) {

List<List<Integer>> result = new LinkedList<>();

helper(candidates, result, new LinkedList<>(), target, 0);

return result;

}

private void helper(int[] candidates, List<List<Integer>> result, List<Integer> list, int target, int index) {

if (target < 0) {

return;

} else if (target == 0) {

result.add(new LinkedList<>(list));

return;

} else if (index >= candidates.length) {

return;

}

list.add(candidates[index]);

helper(candidates, result, list, target - candidates[index], index);

list.remove(list.size() - 1);

helper(candidates, result, list, target, index + 1);

}

// 耗时22ms

public List<List<Integer>> combinationSum2(int[] candidates, int target) {

List<List<Integer>> result = new LinkedList<>();

dfs(candidates, target, 0, result, new LinkedList<Integer>());

return result;

}

private void dfs(int[] candidates, int target, int start, List<List<Integer>> result, List<Integer> list) {

/**

* target小于0退出的条件千万别掉了,题目给的条件是所有数都为正的

*/

if (target < 0) {

return;

}

if (target == 0) {

result.add(new LinkedList<>(list));

return;

}

for (int i = start; i < candidates.length; i++) {

list.add(candidates[i]);

/**

* 注意这里下一个start取i，表示当前数仍可以重复取

*/

dfs(candidates, target - candidates[i], i, result, list);

list.remove(list.size() - 1);

}

}

}