Runtime9 msBeats90.97%
Memory42.1 MBBeats44.5%

class Solution {
    public int countQuadruplets(int[] nums) {
        int res=0, l=nums.length;
        Map<Integer, Integer> count = new HashMap<>();
        count.put(nums[l-1]-nums[l-2], 1);
        for(int b=l-3; b>=1; b--){
            for(int a=b-1; a>=0; a--){
                res+=count.getOrDefault(nums[b]+nums[a], 0);
            }
            for(int d=l-1; d>b; d--){
                count.put(nums[d]-nums[b], count.getOrDefault(nums[d]-nums[b], 0) +1);
            }
        }
        return res;
    }
}