P4622 [COCI 2012/2013 #6] JEDAN

COCI

There are $N$ numbers in a row (their values represent heights). Initially, all numbers are zero. You may choose a continuous segment of numbers with the same height, and increase all of them by $1$ (except the first and last numbers of the segment). The figure below shows two operations:  Now, some numbers cannot be seen clearly, and we use $-1$ to represent them. Based on the remaining numbers, determine how many possible valid arrays there are such that the visible numbers fit exactly the operation rules above.

The first line contains a positive integer $N$, the number of numbers. The next line contains $N$ integers, representing the height of each number in order. A value of $-1$ means it cannot be seen clearly; you may replace it with any number that satisfies the conditions. Let the $i$-th number be $h_i$.

Output one integer: the number of all possible valid arrays modulo $1000000007$.

- $(1 \le N \le 10000)$. - $(-1 \le h_i \le 10000)$. Translated by ChatGPT 5