A.
In rows and columns to build a bipartite graph nodes run Eulerian paths. Note judge whether the communication.
B.
Sub1
Sort violence.
Sub2
Open prefixes and arrays to sort(a+1,a+n+1)change nth_element(a+1,a+k,a+n+1).
Sub3
Just consider \ (l \ in [1,100] , r \ in [n-100, n] \) interval. 10000 sort these intervals. Prefix and no less than open with the Chairman of the tree.
C.
Note expectations can not be multiplied!
Set
\ [DP1 [n-] = \ sum_ {I = 0} ^ n-E (a_i ^ 2) \]
\ [DP2 is [n-] = \ sum_ {I = 0} ^ {n--. 1} \ sum_ {J = 0} ^ {n-1} E (a_i * a_j) \]
\[\sum_{i=0}^n E(a_i^2)\]
\[=\frac{\sum_{i=0}^{n-1}\sum_{j=0}^{n-1}E((a_i+a_j)^2)}{n^2}\]
\[=\frac{2\sum_{i=0}{n-1}E(a_i^2)}{n}+\frac{2\sum_{i=0}^{n-1}\sum_{j=0}^{n-1}E(a_i*a_j)}{n^2}\]
\[=\frac{2*dp1[i-1]}{n}+\frac{2*dp2[i]}{n^2}\]
\[\sum_{i=0}^{n-1}\sum_{j=0}^{n-1} E(a_i*a_j)\]
\[=\sum_{i=0}^{n-2}\sum_{j=0}^{n-2} E(a_i*a_j)+E(a_{n-1}^2)+2\sum_{i=0}^{n-2} E(a_i*a_{n-1})\]
\[=\sum_{i=0}^{n-2}\sum_{j=0}^{n-2} E(a_i*a_j)+E(a_{n-1}^2)+2\sum_{i=0}^{n-2} E(a_i*\frac{2\sum_{j=0}^{n-2} a_j}{n-1})\]
\[=\sum_{i=0}^{n-2}\sum_{j=0}^{n-2} E(a_i*a_j)+E(a_{n-1}^2)+\frac{4*E(\sum_{i=0}^{n-2} a_i*\sum_{j=0}^{n-2} a_j)}{n-1}\]
\[=\frac{n+3}{n-1}*dp2[n-1]+dp1[n-1]\]