Time Limit Exceeded

• Huge input/output? Did you try using scanf, printf?
• Huge input/output? Did you try fast_input_output?
• Too many lines to print? Use #define endl '\n'
• Interactive Problems? Did you forget to flush the output?

Class

• Compilation error? Did you forget the semi colon?
• Compilation error? Did you forget to make the functions/constructor public?
• Compilation error? Did you forget to override a virtual function?
• Compilation error? Did you forget to mention the scope of the methods?

Strings

• TLE? Consider using an array of size 26/256 instead of a map.

Graphs

• Is the graph undirected ? Did you forget to add both sides of the edge in the adjacency list?
• Is the graph directed ? Did you accidentally add an undirected edge instead of a directed one?
• Is the input one-based? Did you forget to decrement everything while taking the input?
• Is the input one-based? Did you forget to increment everything while printing the output?
• Reusing a visited array / Re-running a DFS/BFS? Did you re-initialize everything?
• Have you wondered over the fact that the graph can be disconnected? What about isolated vertices?
• Does your code handle cycles in the graph?
• Take care of cycles of length 2 (Non existent in undirected graphs).
• Getting a TLE while running BFS? Did you forget to pop the queue?

Trees

• Did you forget to convert everything to zero based indexing?
• DP on Trees? Did you forget the condition parent == child? (+2)

DFS

• Recursive call ? Did you call source again instead of child?

Sieve

• Did you start the outer loop from 2 instead of 1?

Bitwise

• Don't mixup |, or