【Hot100】LeetCode—39. 组合总和

目录

• 1- 思路
• 2- 实现
• • ⭐39. 组合总和——题解思路
• 3- ACM 实现

• 题目连接：39. 组合总和

1- 思路

注意如果借助 startIndex 实现，理解 startIndex 的含义

• 在本题目中，由于同一个元素可以重复选取，因此 startIndex 在传递的过程中，不需要进行 +1 操作，传递的值为i

2- 实现

⭐39. 组合总和——题解思路

class Solution {

    List<List<Integer>> res = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        backTracing(candidates, target, 0, 0);
        return res;
    }

    // 1. 回溯参数及返回值
    public void backTracing(int[] candidates, int target, int nowSum, int startIndex) {

        if (nowSum > target) {
            return;
        }

        // 2. 结束条件
        if (nowSum == target) {
            res.add(new ArrayList(path));
            return;
        }

        // 回溯
        for (int i = startIndex; i < candidates.length; i++) {
            nowSum += candidates[i];
            path.add(candidates[i]);
            backTracing(candidates, target, nowSum, i);
            nowSum -= candidates[i];
            path.remove(path.size() - 1);
        }
    }
}

3- ACM 实现

public class combination {


    static List<List<Integer>> res = new ArrayList<>();
    static List<Integer> path = new ArrayList<>();
    public static List<List<Integer>> combine(int[] candidates,int target){
        // 回溯
        backTracing(candidates,target,0,0);
        return res;
    }

    public static void backTracing(int[] candidates,int target,int nowSum,int startIndex){

        // 终止条件
        if(nowSum>target){
            return;
        }
        if(nowSum==target){
            res.add(new ArrayList<>(path));
        }

        //回溯
        for(int i = startIndex;i<candidates.length;i++){
            nowSum += candidates[i];
            path.add(candidates[i]);
            backTracing(candidates,target,nowSum,i);
            nowSum -= candidates[i];
            path.remove(path.size()-1);
        }
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String input = sc.nextLine();
        input = input.replace("[","").replace("]","");
        String[] parts = input.split(",");
        int[] nums = new int[parts.length];
        for(int i = 0 ; i < parts.length;i++){
            nums[i] = Integer.parseInt(parts[i]);
        }
        System.out.println("输入target");
        int target = sc.nextInt();
        System.out.println("结果是"+combine(nums,target).toString());

    }
}