n数之和（力扣）

三数之和

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        return threeSumTarget(nums,0);
    }

    public List<List<Integer>> threeSumTarget(int[] nums,int target){
        Arrays.sort(nums);
        int n=nums.length;
        List<List<Integer>> res=new ArrayList<>();
        for(int i=0;i<n;i++){
            List<List<Integer>> sub=twoSumTarget(nums,i+1,target-nums[i]);
            for(List<Integer> arr:sub){
                arr.add(nums[i]);
                res.add(arr);
            }
            while(i<n-1&&nums[i]==nums[i+1]) i++;
        }
        return res;
    }
    
    public List<List<Integer>> twoSumTarget(int[] nums,int start,int target){
        int lo=start,hi=nums.length-1;
        List<List<Integer>> res=new ArrayList<>();
        while(lo<hi){
            int sum = nums[lo] + nums[hi];
            int left = nums[lo], right = nums[hi];
            if (sum < target) {
                while (lo < hi && nums[lo] == left) lo++;
            } else if (sum > target) {
                while (lo < hi && nums[hi] == right) hi--;
            } else {
                res.add(new ArrayList(Arrays.asList(left,right)));
                while (lo < hi && nums[lo] == left) lo++;
                while (lo < hi && nums[hi] == right) hi--;
            }
        } 
        return res;
    }
}

n数之和 

class Solution {
    public List<List<Integer>> fourSum(int[] nums, long target) {
        Arrays.sort(nums);
        return nSumTarget(nums,4,0,target);
    }

    public List<List<Integer>> nSumTarget(int[] nums,int n,int start,long target){
        int sz=nums.length;
        List<List<Integer>> res=new ArrayList<>();
        if (n < 2 || sz < n) return res;
        if(n==2){
            // 双指针那一套操作
            int lo = start, hi = sz - 1;
            while (lo < hi) {
                int sum = nums[lo] + nums[hi];
                int left = nums[lo], right = nums[hi];
                if (sum < target) {
                    while (lo < hi && nums[lo] == left) lo++;
                } else if (sum > target) {
                    while (lo < hi && nums[hi] == right) hi--;
                } else {
                    res.add(new ArrayList(Arrays.asList(left,right)));
                    while (lo < hi && nums[lo] == left) lo++;
                    while (lo < hi && nums[hi] == right) hi--;
                }
            }
        }else{
            for(int i=start;i<sz;i++){
                List<List<Integer>> sub=nSumTarget(nums,n-1,i+1,target-nums[i]);
                for(List<Integer> arr:sub){
                    //(n-1)sum加上nums[i]即nsum
                    arr.add(nums[i]);
                    res.add(arr);
                }
                while(i<sz-1&&nums[i]==nums[i+1]) i++;
            }
        }
        return res;
    }
  
}