【LeeCode】39. 组合总和

【题目描述】

给你一个 无重复元素 的整数数组 ​​candidates​​​ 和一个目标整数 ​​target​​​ ，找出 ​​candidates​​​ 中可以使数字和为目标数 ​​target​​ 的 所有 不同组合 ，并以列表形式返回。你可以按 任意顺序 返回这些组合。

​​candidates​​ 中的 同一个 数字可以 无限制重复被选取 。如果至少一个数字的被选数量不同，则两种组合是不同的。 

对于给定的输入，保证和为 ​​target​​​ 的不同组合数少于 ​​150​​ 个。

​​https://leetcode.cn/problems/combination-sum/?favorite=2cktkvj​​

【示例】

【代码1】

​​学习参考​​

package com.company;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

class Solution2 {
    List<List<Integer>> res = new ArrayList<>();
    List<Integer> combines = new ArrayList<>();
    int sum = 0;

    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        Arrays.sort(candidates);
        backtrack(0, candidates, target);
        System.out.println(res.toString());
        return res;
    }

    private void backtrack(int idx, int[] candidates, int target) {
        if(sum == target){
            res.add(new ArrayList<>(combines));
            return;
        }

        for (int i = idx; i < candidates.length && sum + candidates[i] <= target; ++i){
            sum += candidates[i];
            combines.add(candidates[i]);
            // 这里以sum为变量, 进行递归, 此时的i是不变的，可重复叠加
            // 如果sum + candicates[i] > target 则退出循环， 回到第一次的结果
            backtrack(i, candidates, target);    // 如果这里的i改成idx 可全排列所有结果
            // sum + candicates[i] > target, 移除最后一个数
            combines.remove(combines.size() - 1);
            sum -= candidates[i];
        }
    }
}
public class Test {
    public static void main(String[] args) {
        int[] candidates = {2,3,6,7};
        int target = 7;
        new Solution2().combinationSum(candidates, target);
    }
}

【代码2】

回溯 + List排序 + Set去重 ​​学习参考​​

package com.company;
import java.util.*;
import java.util.stream.Collectors;

class Solution2 {
    private List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        dfs(candidates, 0, target, new Stack<>());
        return res;
    }

    private void dfs(int[] candidates,int start, int target, Stack<Integer> stack) {
        if (target == 0){
           res.add(new ArrayList<>(stack));
        }else if (target >= 2){
            for (int i = start; i < candidates.length; i++) {
                int num = candidates[i];
                stack.push(num);
                dfs(candidates, i, target - num, stack);
                stack.pop();
            }
        }
    }

}
public class Test {
    public static void main(String[] args) {
        int[] candidates = {2,3,6,7};
        int target = 7;
        System.out.println(new Solution2().combinationSum(candidates, target).toString());
    }
}

【代码3】

​​学习参考​​

import java.util.*;

/**
 * https://leetcode.cn/problems/combination-sum/solutions/406516/zu-he-zong-he-by-leetcode-solution/
 */
public class Solution {
    private static Set<List<Integer>> res = new HashSet<>();
    public static void main(String[] args) {
        int[] arr = {2, 3, 6, 7};
        int target = 7;
        combinationSum(arr, target);
        System.out.println(res.toString());
    }

    public static List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> list = new ArrayList<>();
        dfs(candidates, target, new Stack<>());
        return list;
    }

    private static void dfs(int[] candidates, int target, Stack<Integer> stack) {
        if (target == 0){
            List<Integer> list = new ArrayList<>(stack);
            Collections.sort(list);
            res.add(list);
        }else if(target >= 2){
            for(int num: candidates){
                stack.push(num);
                dfs(candidates, target - num, stack);
                stack.pop();
            }
        }
    }
}

【代码4】

​​学习参考​​

回溯 + 枝减

package com.company;
import java.util.*;

class Solution2 {

    private Set<List<Integer>> res = new HashSet<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        dfs(candidates, target, new Stack<>());
        return new ArrayList<>(res);
    }

    public void dfs(int[] candidates, int target, Stack<Integer> stack) {
        if (target == 0){
            List<Integer> list = new ArrayList<>(stack);
            // set 去重用
            Collections.sort(list);
            res.add(list);
        }else if(target >= 2){  //题中规定元素不会小于 2
            for (int num : candidates) {
                stack.push(num);
                dfs(candidates, target - num, stack);
                stack.pop();
            }
        }
    }
}
public class Test {
    public static void main(String[] args) {
        int[] candidates = {2,3,6,7};
        int target = 7;
        System.out.println(new Solution2().combinationSum(candidates, target).toString());
    }
}