LeetCode刷题笔录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] 

这题和之前的很多递归题目差不多。在每层递归都把数组元素按顺序加进去一次，然后丢到递归的下一层去。

public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(candidates == null || candidates.length == 0)
            return result;
        Arrays.sort(candidates);
        recursive(candidates, target, 0, result, new ArrayList<Integer>());
        return result;
    }
    
    public void recursive(int[] candidates, int target, int index, List<List<Integer>> result, List<Integer> curr){
        if(target < 0)
            return;
        if(target == 0){
            result.add(new ArrayList<Integer>(curr));
            return;
        }
        for(int i = index; i < candidates.length; i++){
            //add the current element
            curr.add(candidates[i]);
            recursive(candidates, target - candidates[i], i, result, curr);
            //remove the current element
            curr.remove(curr.size() - 1);
        }
    }
}