java刷题--39组合总和

Posted 2021-05-14 Anrys

java刷题--39组合总和

• 题目
• 代码
• 结果

题目

代码

class Solution {
    List<List<Integer>> res;
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        res = new ArrayList<>();
        Arrays.sort(candidates);       
        backtrack(candidates, target, new ArrayList<>(), 0);       
        return res;
    }
    private void backtrack(int[] candidates, int remains, List<Integer> path, int start){
        if(remains == 0){  // 临界条件
            res.add(new ArrayList<>(path));
            return;
        }
        for(int i = start; i < candidates.length; i++){//遍历每一个数
            if(candidates[i] > remains) continue;                           
            if(i > 0 && candidates[i] == candidates[i-1]) continue;//两次剪枝 
            //核心回溯算法代码          
            path.add(candidates[i]);            
            backtrack(candidates, remains - candidates[i], path, i);           
            path.remove(path.size() - 1);
        }
    }
}

**backtrack的公式**
result = []
def backtrack(路径, 选择列表):
    if 满足结束条件:
        result.add(路径)
        return
    
    for 选择 in 选择列表:
        做选择
        backtrack(路径, 选择列表)
        撤销选择

class Solution {
    List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        Arrays.sort(candidates);
        backTrack(candidates, target, 0, new ArrayList<>());
        return res;
    }
    private void backTrack(int[] candidates, int target, int start, ArrayList<Integer> track) {
        if (target == 0) {
            res.add(new ArrayList<>(track));
            return;
        }
        for (int i = start; i < candidates.length; i++) {
            if (target - candidates[i] < 0) break;
            track.add(candidates[i]);
            backTrack(candidates, target - candidates[i], i, track);
            track.remove(track.size() - 1);
        }
    }
}

结果