问题描述
https://leetcode.com/problems/combination-sum/
解决思路
直接看篮子王的视频吧，这里不太好解释，看代码应该也能看个大概懂。
问题解惑
有很多问题，递归如何调用，回溯怎么回溯，一层层回溯的过程。都通过debug理解了，没办法一句句说清楚，这题要多回头看看。
代码
class Solution {
public List<List<Integer>> combinationSum(int[] candidates, int target) {
List<List<Integer>> res = new ArrayList<>();
//corner case
if(candidates.length == 0 || candidates == null || target < 0) return res;
Arrays.sort(candidates);
helper(res, new ArrayList<>(), candidates, target, 0);
return res;
}
public void helper(List<List<Integer>> res, List<Integer> list, int[] candidates, int target, int start){
//backtracking condition
if(target < 0) return;
//one result has been found and add it into res
if(target == 0){
res.add(new ArrayList<>(list));
return;
}
for(int i = start; i < candidates.length; i++){
list.add(candidates[i]);
helper(res, list, candidates, target - candidates[i], i);//recursive function
list.remove(list.size() - 1);
}
}
}
复杂度分析
- 时间复杂度：O(2^n)
- 空间复杂度：O(n)
