These problems usually rely on keen observation and I had fun solving them. Many of these problems also taught me new things. It's mainly personal, others may find use of it too. Will keep updating this post in the future.

R2100:

• F. Ant Colony: https://codeforces.com/contest/474/problem/F

• C. Ciel the Commander: https://codeforces.com/problemset/problem/321/C

• E. Array Shrinking: https://codeforces.com/problemset/problem/1312/E

R1900:

• F. Kirei and the Linear Function: https://codeforces.com/contest/1729/problem/F

• D. Reset K Edges: https://codeforces.com/contest/1739/problem/D

• D. Paths on the Tree: https://codeforces.com/problemset/problem/1746/D

• D. Zigzags: https://codeforces.com/problemset/problem/1400/D

• B. Zuma: https://codeforces.com/problemset/problem/607/B

• D. TediousLee: https://codeforces.com/problemset/problem/1369/D

• C. Guessing the Greatest (hard version): https://codeforces.com/problemset/problem/1486/C2

• E. Replace the Numbers: https://codeforces.com/problemset/problem/1620/E

• E. String Reversal: https://codeforces.com/problemset/problem/1430/E

• D. Score of a Tree https://codeforces.com/problemset/problem/1777/D

R1800:

• D. Maximum AND: https://codeforces.com/contest/1721/problem/D

• B. Unmerge: https://codeforces.com/contest/1381/problem/B

• D. Yet Another Minimization Problem: https://codeforces.com/problemset/problem/1637/D

• A. Extreme Subtraction: https://codeforces.com/problemset/problem/1442/A

• D. Same GCDs: https://codeforces.com/problemset/problem/1295/D

• A. Book: https://codeforces.com/problemset/problem/1572/A

• D. MUH and Cube Walls: https://codeforces.com/problemset/problem/471/D

• D. Rescue Nibel!: https://codeforces.com/problemset/problem/1420/D

R1700:

• A2. Prefix Flip (Hard Version): https://codeforces.com/problemset/problem/1381/A2

• D. Valid BFS?: https://codeforces.com/problemset/problem/1037/D

• D. Ehab the Xorcist: https://codeforces.com/problemset/problem/1325/D

R1600:

• C. Carrying Conundrum: https://codeforces.com/contest/1567/problem/C

• A. Parsa's Humongous Tree: https://codeforces.com/contest/1528/problem/A

• C. Strange Function: https://codeforces.com/contest/1542/problem/C

• A. Linova and Kingdom: https://codeforces.com/problemset/problem/1336/A

• D. Constructing the Array: https://codeforces.com/problemset/problem/1353/D

• C. Carrying Conundrum: https://codeforces.com/problemset/problem/1567/C

  • The base problem is easy, try solving it assuming higher constraints (length of n, 1 < |n| <= 2*10^5, total length of all numbers doesn't exceed 2*10^5). You will need to calculate it as modulo M (i.e M = 1000000007) for such constraints as the result would be too big.

• C. Strange Test: https://codeforces.com/problemset/problem/1632/C

• C. Longest Simple Cycle: https://codeforces.com/problemset/problem/1476/C

R1500:

• Weights Assignment For Tree Edges: https://codeforces.com/contest/1611/problem/D