【7.12】Codeforces Round ＃806 (Div. 4)

E. Mirror Grid

The question ： Given 01 matrix , One operation can reverse one grid . Ask how many operations at least make the rotation of the matrix equal to the original matrix any time .

Ideas ： A formula of rotation is summarized ： If a certain point $(i,j)$ stay $n\times n$ Rotate in the matrix of $90°$ , Then the coordinates are $(j,n-i+1)$ . Pay attention to this question $n$ You can't enumerate too many when it's an odd number .

AC Code ：https://codeforces.com/contest/1703/submission/163868674

G. Good Key, Bad Key

The question ： existing $n$ A box , The first $i$ There are $a_i$ Gold coin . Now you have to press $1$ To $n$ Open these boxes in order . You can open the box with two kinds of keys , There are good keys and bad keys . Each time you use a good key to unpack, it will cost $k$ Gold coins ; Each time you use a bad key to unpack, you won't spend gold coins , But it will halve the gold coins in all unopened boxes, including the box to be opened ( Round down ). first , You don't have any gold coins or keys . During unpacking , You can be in debt . Ask how many gold coins you can hold at most after opening all the boxes .

Answer key ：Codeforces Round #806 (Div. 4)

Ideas ： greedy . If we choose $x$ A bad key , that 1 Analysis shows that the bad key must be the last and continuous use . So enumerate the boundaries of good keys and bad keys , Just update the answer .

AC Code ：https://codeforces.com/contest/1703/submission/164017522