description: Let's call a sequence of integers x_1, x_2, ..., x_k MEX-correct if for all i (1 ≤ i ≤ k) |x_i - \operatorname{MEX}(x_1, x_2, ..., x_i)| ≤ 1 holds. Where \operatorname{MEX}(x_1, ..., x_k) is the minimum non-negative integer that doesn't belong to the set x_1, ..., x_k. For example, \operatorname{MEX}(1, 0, 1, 3) = 2 and \operatorname{MEX}(2, 1, 5) = 0.
  
  You are given an array a consisting of n non-negative integers. Calculate the number of non-empty MEX-correct subsequences of a given array. The number of subsequences can be very large, so print it modulo 998244353. 
  
  Note: a subsequence of an array a is a sequence [a_{i_1}, a_{i_2}, ..., a_{i_m}] meeting the constraints 1 ≤ i_1 < i_2 < ... < i_m ≤ n. If two different ways to choose the sequence of indices [i_1, i_2, ..., i_m] yield the same subsequence, the resulting subsequence should be counted twice (i. e. two subsequences are different if their sequences of indices [i_1, i_2, ..., i_m] are not the same).
  
  Input
  
  The first line contains a single integer t (1 ≤ t ≤ 10^5) — the number of test cases.
  
  The first line of each test case contains a single integer n (1 ≤ n ≤ 5 ⋅ 10^5).
  
  The second line contains n integers a_1, a_2, ..., a_n (0 ≤ a_i ≤ n).
  
  The sum of n over all test cases doesn't exceed 5 ⋅ 10^5.
  
  Output
  
  For each test case, print a single integer — the number of non-empty MEX-correct subsequences of a given array, taken modulo 998244353.
  
  Example
  
  Input
  
  
  4
  3
  0 2 1
  2
  1 0
  5
  0 0 0 0 0
  4
  0 1 2 3
  
  
  Output
  
  
  4
  2
  31
  7
  
  Note
  
  In the first example, the valid subsequences are [0], [1], [0,1] and [0,2].
  
  In the second example, the valid subsequences are [0] and [1].
  
  In the third example, any non-empty subsequence is valid. 
solution: #include <bits/stdc++.h>
  using namespace std;
  using ll = long long;
  const ll mod = (ll)998244353;
  const int N = (int)5e5 + 5;
  const int M = (int)1e6 + 5;
  int _ = 1;
  ll n, ans;
  ll a[N];
  ll num[N], maxn[N];
  void work() {
    ans = 0;
    scanf("%lld", &n);
    for (ll i = 1; i <= n; i++) scanf("%lld", &a[i]);
    for (ll i = 0; i <= n + 1; i++) num[i] = maxn[i] = 0;
    for (ll i = 1; i <= n; i++) {
      if (a[i] - 1 >= 0) ans = (ans + num[a[i] - 1] + maxn[a[i] - 1]) % mod;
      ans = (ans + maxn[a[i]]) % mod;
      ans = (ans + num[a[i] + 1] + maxn[a[i] + 1]) % mod;
      if (a[i] <= 1) ans = (ans + 1) % mod;
      if (a[i] - 1 >= 0) {
        num[a[i] - 1] = num[a[i] - 1] * 2 % mod;
        num[a[i] - 1] = (num[a[i] - 1] + maxn[a[i] - 1]) % mod;
      }
      num[a[i] + 1] = num[a[i] + 1] * 2 % mod;
      maxn[a[i] + 1] = maxn[a[i] + 1] * 2 % mod;
      maxn[a[i] + 1] = (maxn[a[i] + 1] + maxn[a[i]]) % mod;
      if (!a[i])
        maxn[1]++;
      else if (a[i] == 1)
        num[0]++;
    }
    printf("%lld\n", ans);
  }
  int main() {
    scanf("%d", &_);
    while (_--) {
      work();
    }
    return 0;
  }
  