Count Special Triplets solution and explanation

class Solution:
    def specialTriplets(self, nums: List[int]) -> int:
        left = Counter()
        right = Counter(nums)
        ans = 0
        mod = 10**9 + 7
        for x in nums:
            right[x] -= 1
            ans = (ans + left[x * 2] * right[x * 2] % mod) % mod
            left[x] += 1
        return ans

class Solution {
    public int specialTriplets(int[] nums) {
        Map<Integer, Integer> left = new HashMap<>();
        Map<Integer, Integer> right = new HashMap<>();
        for (int x : nums) {
            right.merge(x, 1, Integer::sum);
        }
        long ans = 0;
        final int mod = (int) 1e9 + 7;
        for (int x : nums) {
            right.merge(x, -1, Integer::sum);
            ans = (ans + 1L * left.getOrDefault(x * 2, 0) * right.getOrDefault(x * 2, 0) % mod)
                % mod;
            left.merge(x, 1, Integer::sum);
        }
        return (int) ans;
    }
}

class Solution {
public:
    int specialTriplets(vector<int>& nums) {
        unordered_map<int, int> left, right;
        for (int x : nums) {
            right[x]++;
        }
        long long ans = 0;
        const int mod = 1e9 + 7;
        for (int x : nums) {
            right[x]--;
            ans = (ans + 1LL * left[x * 2] * right[x * 2] % mod) % mod;
            left[x]++;
        }
        return (int) ans;
    }
};

function specialTriplets(nums: number[]): number {
    const left = new Map<number, number>();
    const right = new Map<number, number>();
    for (const x of nums) {
        right.set(x, (right.get(x) || 0) + 1);
    }
    let ans = 0;
    const mod = 1e9 + 7;
    for (const x of nums) {
        right.set(x, (right.get(x) || 0) - 1);
        const lx = left.get(x * 2) || 0;
        const rx = right.get(x * 2) || 0;
        ans = (ans + ((lx * rx) % mod)) % mod;
        left.set(x, (left.get(x) || 0) + 1);
    }
    return ans;
}

func specialTriplets(nums []int) int {
	left := make(map[int]int)
	right := make(map[int]int)
	for _, x := range nums {
		right[x]++
	}
	ans := int64(0)
	mod := int64(1e9 + 7)
	for _, x := range nums {
		right[x]--
		ans = (ans + int64(left[x*2])*int64(right[x*2])%mod) % mod
		left[x]++
	}
	return int(ans)
}

use std::collections::HashMap;

impl Solution {
    pub fn special_triplets(nums: Vec<i32>) -> i32 {
        let mut left: HashMap<i32, i64> = HashMap::new();
        let mut right: HashMap<i32, i64> = HashMap::new();

        for &x in &nums {
            *right.entry(x).or_insert(0) += 1;
        }

        let modulo: i64 = 1_000_000_007;
        let mut ans: i64 = 0;

        for &x in &nums {
            if let Some(v) = right.get_mut(&x) {
                *v -= 1;
            }

            let t = x * 2;

            let l = *left.get(&t).unwrap_or(&0);
            let r = *right.get(&t).unwrap_or(&0);

            ans = (ans + (l * r) % modulo) % modulo;

            *left.entry(x).or_insert(0) += 1;
        }

        ans as i32
    }
}