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CF2143A.All Lengths Subtraction
All Lengths Subtraction
You are given a permutation of length .
You must perform exactly one operation for each integer from 1 up to in that order:
- Choose a subarray of of length exactly , and subtract 1 from every element in that subarray.
After completing all operations, your goal is to have all elements of the array equal to zero.
Determine whether it is possible to achieve this.
A permutation of length is an array consisting of distinct integers from to in arbitrary order. For example, is a permutation, but is not a permutation ( appears twice in the array), and is also not a permutation ( but there is in the array).
An array is a subarray of an array if can be obtained from by the deletion of several (possibly, zero or all) elements from the beginning and several (possibly, zero or all) elements from the end.
Input
Each test contains multiple test cases. The first line contains the number of test cases (). The description of the test cases follows.
The first line contains the value () — the length of the permutation.
The second line contains () — the permutation itself.
Output
For each test case, output YES if it is possible to make all elements of the array equal to after performing all the operations; otherwise, output NO.
You can output the answer in any case (upper or lower). For example, the strings "yEs", "yes", "Yes", and "YES" will be recognized as positive responses.
Note
For the first test case, we can proceed as follows:
- : Choose the subarray . The array becomes .
- : Choose the subarray . The array becomes .
- : Choose the subarray . The array becomes .
- : Choose the subarray . The array becomes .
Thus, we have successfully reduced the entire array to zeros, so the answer is YES.
For the second test case, it can be shown that it is impossible to reduce all elements to zero.
For the third test case, the process is as follows:
$$[2, 4, \boldsymbol{5}, 3, 1] \rightarrow [2, \boldsymbol{4, 4}, 3, 1] \rightarrow [2, \boldsymbol{3, 3, 3}, 1] \rightarrow [\boldsymbol{2, 2, 2, 2}, 1] \rightarrow [\boldsymbol{1, 1, 1, 1, 1}] \rightarrow [0, 0, 0, 0, 0].$$The bolded values indicate the subarrays from which we subtract at each step.
For the fourth test case, it can also be proven that it is impossible to make all values zero.
Samples
4
4
1 3 4 2
5
1 5 2 4 3
5
2 4 5 3 1
3
3 1 2
YES
NO
YES
NO
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