Combination Sum

Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

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

Note:

• All numbers (including target) will be positive integers.

• The solution set must not contain duplicate combinations.

Example 1:

Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
  [7],
  [2,2,3]
]

Example 2:

Input: candidates = [2,3,5], target = 8,
A solution set is:
[
  [2,2,2,2],
  [2,3,3],
  [3,5]
]

class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> res = new ArrayList<>();
        
        if(candidates == null || candidates.length == 0)
            return res;
        // Arrays.sort(candidates);
        backtracking(candidates,res,new ArrayList<>(),target,0);
        
        return res;
    }
    
    public void backtracking(int[] candidates, List<List<Integer>> res, List<Integer> list, int remain,int start){
        if(remain < 0)
            return;
        
        if(remain == 0){
            res.add(new ArrayList<>(list));
            return;
        }
        
        for(int i = start; i < candidates.length; i++){
            list.add(candidates[i]);
            backtracking(candidates,res,list,remain-candidates[i],i);
            
            list.remove(list.size()-1);
        }
            
    }
}

因为都是搜索题，所以搜索的时间复杂度是O(解的个数 * 每个解生成的复杂度) combination sum 这个题目因为有target存在所以并不知道解的个数，但是解的个数最多是子集的个数2^n, 产生每个解的最坏复杂度n，所以我们可以粗略的估计时间复杂度是O(2^n * n)

Combination sum 1 and 2|一亩三分地刷题版一亩三分地 1Point3Acres

Combinations和Subset时间复杂度比较[Leetcode]|一亩三分地刷题版一亩三分地 1Point3Acres