Educational Codeforces Round 90 Postmortem

What went well

• B, C, D took me 5 minutes, 9 minutes, and 24 minutes respectively.

What went poorly

• A took 21 minutes (drew a huge blank; no, I did not wake up late), which was a massive penalty, since it increases the penalties for B, C, and D.
• Couldn't get E in an hour.
  

Where I got lucky

• D was on the easier side.
  

Codeforces Round #652 (Div. 2) Postmortem

What went well

• 0:00 submission for A!
• Got ABCD (albeit slowly).

What went poorly

• Incurred one wrong answer for B and C each (and took a long time too).
• Took half an hour to recognize the recursion for the tree structure in D.
• Thought that the residues of numbers compare the same way as the the numbers themselves (i.e. a > b implies (a % m) > (b % m)…no, just no).

Where I got lucky

• Got D with 9 minutes to go, which involved a slight wild guess.
  

Codeforces Round #651 (Div. 2) Postmortem

What went well

• Decently fast ABC (0:01, 0:10, 0:23).
  

What went wrong

• Walled by D. I didn't recognize that binary search works because I didn't know how to verify that a subsequence with max less than or equal to a target value was doable in O(N).
• I thought incorrectly (yet again) that O(N lg N) is too slow for N <= 10**6, so I tried something silly to find the minimum # subsequences of the form (01)+ or (10)+, when a straightforward O(N lg N) would have worked.
  

Where I got lucky

• Gained rating!
  

Bugforces (ft. senile logarithmic exponentiation)

Sad things happen when you lose your mind:

using i64 = long long;

// Precondition: (base, exponent) won't cause overflow
i64
my_pow(const i64 base, const i64 exponent)
{
  i64 accumulator = 0;
  for (i64 exponent2 = exponent, current_power = base; exponent2 > 0;
      exponent2 >>= 1, current_pow *= current_pow)
  {
    if (exponent2 & 1)
    {
      accumulator += current_power;
    }
  }
  return accumulator;
}

Codeforces Round #647 (Div. 2) Postmortem

What went well

• Got B in 5 minutes -- straightforward problem.
  
• Got C in 5 minutes -- another straightforward problem.
• Got D in 20 minutes, once I figured out how to read. :-P
• New personal best!!!
  

What went poorly

• Wrote an extremely clumsy solution for A because I decided after a brief tradeoff analysis to match the problem statement as close as possible instead of thinking about a better implementation
  
• IlliterateForces strikes again -- misread D, so submitted a solution that solves a similar problem, which cost ~20min and a submission penalty.
• Couldn't get E.
  

Where I got lucky

• Got extremely lucky with B and C. I have been struggling with Bs recently; this B's 5 minute solve time thankfully bucks the trend. My previous fastest C solve time was ~15 minutes.