Score : 400 points
There are N empty boxes arranged in a row from left to right. The integer i is written on the i-th box from the left (1 \leq i \leq N).
For each of these boxes, Snuke can choose either to put a ball in it or to put nothing in it.
We say a set of choices to put a ball or not in the boxes is good when the following condition is satisfied:
Does there exist a good set of choices? If the answer is yes, find one good set of choices.
Input is given from Standard Input in the following format:
N a_1 a_2 ... a_N
If a good set of choices does not exist, print -1
.
If a good set of choices exists, print one such set of choices in the following format:
M b_1 b_2 ... b_M
where M denotes the number of boxes that will contain a ball, and b_1,\ b_2,\ ...,\ b_M are the integers written on these boxes, in any order.
3 1 0 0
1 1
Consider putting a ball only in the box with 1 written on it.
Thus, the condition is satisfied, so this set of choices is good.
5 0 0 0 0 0
0
Putting nothing in the boxes can be a good set of choices.