39. 组合总和(leetcode)

https://leetcode.cn/problems/combination-sum/description/

两种搜索思路

一种是选或不选,搜索树是一颗二叉树
另一种是选哪个数,是一个n叉树

二叉

class Solution {
    
    List<List<Integer>> res = new ArrayList<>();
    int target;
    int[] nums;
    List<Integer> path= new ArrayList<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        this.nums=candidates;
        this.target=target;
        dfs(0,0);
        return res;
    }

    void dfs(int step,int sum)
    {
        if(step>=nums.length || sum>target)return;
        if(sum==target)
        {
            res.add(new ArrayList(path));
            return;
        }
        // 不选
        dfs(step+1,sum);
        // 选
        path.add(nums[step]);
        dfs(step,sum+nums[step]);
        path.remove(path.size()-1);
    }

}

 

n叉

class Solution {
    
    List<List<Integer>> res = new ArrayList<>();
    int target;
    int[] nums;
    List<Integer> path= new ArrayList<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        this.nums=candidates;
        this.target=target;
        dfs(0,0);
        return res;
    }

    void dfs(int startNum,int sum)
    {
        if(sum>target)return;
        if(sum==target)
        {
            res.add(new ArrayList(path));
            return;
        }
        // 多分支
        for(int i=startNum;i<=nums.length-1;i++)
        {
            path.add(nums[i]);
            dfs(i,sum+nums[i]);
            path.remove(path.size()-1);
        }
    }

}