Arithmetic Slices II - Subsequence.cpp

/* 

    Solution by Rahul Surana
    
    ***********************************************************

Given an integer array nums, return the number of all the arithmetic subsequences of nums.

A sequence of numbers is called arithmetic if it consists of at least three elements and 
if the difference between any two consecutive elements is the same.

For example, [1, 3, 5, 7, 9], [7, 7, 7, 7], and [3, -1, -5, -9] are arithmetic sequences.
For example, [1, 1, 2, 5, 7] is not an arithmetic sequence.
A subsequence of an array is a sequence that can be formed by removing some elements (possibly none) of the array.

For example, [2,5,10] is a subsequence of [1,2,1,2,4,1,5,10].
The test cases are generated so that the answer fits in 32-bit integer.


    ***********************************************************
*/

class Solution {
public:
    int numberOfArithmeticSlices(vector<int>& nums) {
        int ans = 0, n = nums.size();
        vector<unordered_map<int,int>> dp(n);
        for(int i = 1; i < n; i++){
            for(int j = 0; j < i; j++){
                if((nums[i]-nums[j]*1L) > INT_MAX || (nums[i]-nums[j]*1L) <INT_MIN)continue;
                int dif = (nums[i]-nums[j]);
                dp[i][dif] += 1;
                if(dp[i].count(dif)){
                    dp[i][dif] += dp[j][dif];
                    ans += dp[j][dif];
                }
            }
        }
        return ans; 
    }
};