[leetcode]39. Combination Sum(Java)

leetcode：https://leetcode.com/problems/combination-sum/#/description

Given a set of candidate numbers (C) (without duplicates) 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.
• 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]
]

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JavaCode：

package go.jacob.day621;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * [leetcode]39. Combination Sum
 * 
 * @author Administrator
 *
 */
public class Demo3 {
	public List> combinationSum(int[] candidates, int target) {
		List> res = new ArrayList>();
		if (candidates == null || candidates.length < 1)
			return res;
		Arrays.sort(candidates);

		backtrack(candidates, target, res, new ArrayList(), 0);

		return res;
	}

	// 如果不加参数start，会出现：input:[2,3,6,7]7
	// 结果：[[2,2,3],[2,3,2],[3,2,2],[7]]instead of [[2,2,3],[7]]的情况
	private void backtrack(int[] nums, int target, List> res, ArrayList list, int start) {
		if (target < 0)
			return;
		if (target == 0) {
			res.add(list);
			return;
		}
		//加上target >= nums[i]：33ms to 23ms
		for (int i = start; i < nums.length && target >= nums[i]; i++) {
			list.add(nums[i]);
			backtrack(nums, target - nums[i], res, new ArrayList(list), i);
			list.remove(list.size() - 1);
		}
	}
}