description: You are given an array of n positive integers a_1, a_2, …, a_n. Your task is to calculate the number of arrays of n positive integers b_1, b_2, …, b_n such that: 
  
    * 1 ≤ b_i ≤ a_i for every i (1 ≤ i ≤ n), and 
    * b_i ≠ b_{i+1} for every i (1 ≤ i ≤ n - 1). 
  
  
  
  The number of such arrays can be very large, so print it modulo 998 244 353.
  
  Input
  
  The first line contains a single integer n (1 ≤ n ≤ 2 ⋅ 10^5) — the length of the array a.
  
  The second line contains n integers a_1, a_2, …, a_n (1 ≤ a_i ≤ 10^9).
  
  Output
  
  Print the answer modulo 998 244 353 in a single line.
  
  Examples
  
  Input
  
  
  3
  2 2 2
  
  
  Output
  
  
  2
  
  Input
  
  
  2
  2 3
  
  
  Output
  
  
  4
  
  Input
  
  
  3
  1 1 1
  
  
  Output
  
  
  0
  
  Note
  
  In the first test case possible arrays are [1, 2, 1] and [2, 1, 2].
  
  In the second test case possible arrays are [1, 2], [1, 3], [2, 1] and [2, 3].
solution: #include <bits/stdc++.h>
  using namespace std;
  int a[200005], dp[200005], stk[200005], top = 0;
  int sum[200005];
  int Sum(int x, int y) {
    if (!x) return sum[y];
    if ((y - x + 1) & 1) return (sum[y] + sum[x - 1]) % 998244353;
    return (sum[y] - sum[x - 1] + 998244353) % 998244353;
  }
  int main() {
    int n, i;
    scanf("%d", &n);
    for (i = 1; i <= n; i++) scanf("%d", a + i);
    dp[0] = 1;
    sum[0] = 1;
    for (i = 1; i <= n; i++) {
      while (top && a[stk[top]] >= a[i]) top--;
      dp[i] = 1ll * a[i] * Sum(stk[top], i - 1) % 998244353;
      if (stk[top]) {
        if ((i - stk[top]) & 1)
          dp[i] = (dp[i] - dp[stk[top]] + 998244353) % 998244353;
        else
          dp[i] = (dp[i] + dp[stk[top]]) % 998244353;
      }
      sum[i] = (dp[i] - sum[i - 1] + 998244353) % 998244353;
      stk[++top] = i;
    }
    printf("%d\n", dp[n]);
    return 0;
  }
  