p39

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LeetCode p39 Combination Sum 题解

1.题目：

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.
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]
]

题意：

给一个数组，和一个数k，从你这个数组中选取一些数（可以重复），是和为k。
输出所有的组合方式。

2.解题思路:

见代码，递归。
先把数组排序。

3.代码

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public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
    List<List<Integer>> ans = new ArrayList<List<Integer>>();
		Arrays.sort(candidates);
		List<Integer> list = new ArrayList<Integer>();
		combinationSum(candidates, target, 0, ans, list);

		return ans;
	}

	public void combinationSum(int[] candidates, int target, int start,
			List<List<Integer>> ans, List<Integer> list) {

		if (target == 0) {
			ans.add(new ArrayList<Integer>(list));
			// System.out.println(list);
			return;
		}

		for (int i = start; i < candidates.length
				&& target - candidates[i] >= 0; i++) {
			list.add(candidates[i]);
			combinationSum(candidates, target - candidates[i], i, ans, list);
			list.remove(list.size() - 1);
			
		}

	}
}

4.一些总结：