Q57 Arithmetic Slices II - Subsequence - LeetCode 446

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Sat Apr 01 2023 13:34:09 GMT+0000 (Coordinated Universal Time)

Saved by @Ayush_dabas07

class Solution {
    public int numberOfArithmeticSlices(int[] nums) {
        /* Q57 of dp playlist , every cell denotes total no. of subarrays APs 
        of size >=2 ending at that ith element. here dp is an array of HM
        
        we make a HM[] to store common difference -> no.of Ap for all elements
        */
        int n = nums.length , ans = 0;
        //making HM
        HashMap<Integer,Integer>[] map = new HashMap[n];

        //iniitalizing all hashmap in hashmap[]
        for(int i = 0 ; i < n ; i++)
        map[i] = new HashMap<>();

        //common approach of LIS
        for(int i = 1 ; i < n ;i++){

            for(int j = i-1 ; j >= 0 ; j--){
                long cd = (long)nums[i]-(long)nums[j];

            //edge case if cd goes out of integer range
            if(cd <= Integer.MIN_VALUE || cd >= Integer.MAX_VALUE)continue;
            
        /*we traverse the array in LIS fashion to store subarray ap of size >=2
        for each element if at any moment we find that the common difference 
        found by nums[i] - nums[j] already exists that means current element 
        is going to be the 3rd or more member of the AP so now total no. of
        AP's  of length >=3 increases by 1 and we add to the answer */
                if(map[j].containsKey((int)cd)){
                    int AP = map[j].get((int)cd);

                    map[i].put((int)cd , map[i].getOrDefault((int)cd,0) + AP+1);
                    ans += AP;
                }
        /* if its a new (cd then we just add it to the map and they become a 2
        sized ap , if in the future some number has same common difference it
        will add to this AP 
        */
                else
                map[i].put((int)cd , map[i].getOrDefault((int)cd,0)+1);

            }
        }
        return ans;
    }
}

https://leetcode.com/problems/arithmetic-slices-ii-subsequence/description/

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