1.HashSet

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> millionYuanList = new ArrayList<>();
        
        if(nums.length < 3) {
            return millionYuanList;
        }

        Arrays.sort(nums);     
        for(int i = 0; i < nums.length; i++) {
            
            if(nums[i] > 0) break;
            Integer first = nums[i];    

            if(i > 0 && nums[i] == nums[i - 1]) continue;

            Set<Integer> set = new HashSet<>();
            for(int j = i + 1; j < nums.length; j++) {
                int third = nums[j];
                int second = -(first + third);    

                if(set.contains(second)) {
                    millionYuanList.add(new ArrayList<>(Arrays.asList(first, third, -(first + third))));

                    while(j < nums.length - 1 && nums[j] == nums[j + 1]) j++;
                }
                set.add(third);
            }
        }
        return millionYuanList;
    }
}

2.HashMap

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);//排序 为了后面去重
        HashMap<Integer,Integer> hm=new HashMap<>();
        List<List<Integer>> list=new ArrayList<List<Integer>>();
        for(int i=0;i<nums.length;i++){
            hm.put(nums[i],i);//根据containsKey选择nums[i]为key
        }
        for(int i=0;i<nums.length;i++){
            if(nums[i]>0)break;
            if(i>0 && nums[i]==nums[i-1])continue;//去重
            int target=nums[i]*(-1);
            for(int j=i+1;j<nums.length;j++){
                if(j>i+1 && nums[j]==nums[j-1])continue;//去重
                int k=target-nums[j];
                if(hm.containsKey(k)){
                    if(hm.get(k)>j ){
                    List<Integer> is=new ArrayList<Integer>();   
                    is.add(nums[i]);
                    is.add(nums[j]);
                    is.add(k);
                    list.add(is);          
                    }
                }    
            }
                            
        }
        return list;

    }
}

3.什么都不用

class Solution{
    public List<List<Integer>> threeSum(int[] nums){
        int n = nums.length;
        Arrays.sort(nums);
        List<List<Integer>> ans = new ArrayList<List<Integer>>();
        for(int i=0;i<n;i++){
            if(i>0 && nums[i]==nums[i-1]) continue;
            int k = n-1;
            int target = -nums[i];
            for(int j = i+1;j<n;j++){
                if(j>i+1 && nums[j]==nums[j-1]) continue;
                while(j<k && nums[j]+nums[k]>target){
                    k--;
                }
                if(j==k) break;
                if(nums[j]+nums[k]==target){
                    List<Integer> list = new ArrayList<Integer>();
                    list.add(nums[i]);
                    list.add(nums[j]);
                    list.add(nums[k]);
                    ans.add(list);
                }
            }
        }
        return ans;
    }
}

4.双指针

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        Arrays.sort(nums);

        for (int i = 0; i < nums.length; i++) {
            if (nums[i] > 0) {
                return result;
            }

            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }

            int left = i + 1;
            int right = nums.length - 1;
            while (right > left) {
                int sum = nums[i] + nums[left] + nums[right];
                if (sum > 0) {
                    right--;
                } else if (sum < 0) {
                    left++;
                } else {
                    result.add(Arrays.asList(nums[i], nums[left], nums[right]));

                    while (right > left && nums[right] == nums[right - 1]) right--;
                    while (right > left && nums[left] == nums[left + 1]) left++;
                    
                    right--; 
                    left++;
                }
            }
        }
        return result;
    }
}