数组类39--组合总和（M）

image.png

AC代码

import java.util.*;
class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> ans = new ArrayList<>();
        if(candidates == null || candidates.length == 0) {
            return ans;
        }
        ArrayList<Integer> tmp = new ArrayList<>();
        backTrack(candidates, target, 0, tmp, ans);
        return ans;
    }
    private void backTrack(int[] candidates, int target, int nowStart, ArrayList<Integer> tmp, 
                           List<List<Integer>> ans) {
        if(target < 0) {
            return;
        }
        if(target == 0) {
            ans.add(new ArrayList<>(tmp));
            return;
        }
        for(int start = nowStart; start < candidates.length; start++) {
            tmp.add(candidates[start]);
            backTrack(candidates, target - candidates[start], start, tmp, ans);
            tmp.remove(tmp.size() - 1);
        }
        
    }
}

精髓
其实是DFS，所谓的回溯法，为一个满N叉树，但是要按数组的形式实现，这里剪枝其实就是target<0