Everyone gets bugs. The difference between a pro and a beginner is how fast they find them.

Never use cout to find where your code fails, because it might mess up your program output. Use cerr (Character Error) instead. It prints to the terminal but is ignored by the online judge.

int target = 10;
cerr << "DEBUG: Current value of target is: " << target << "\n";

1. Integer Overflow: If a problem says $N \times M$ and both are $10^9$, the product is $10^{18}$. An int only goes up to $2 \times 10^9$. You MUST use long long.
2. Off-by-One: Does your loop start at 0 or 1? Does it go to i < n or i <= n? This is the most common bug in history.
3. Array Out of Bounds: Accessing arr[10] when your array size is 10 (valid indices are 0 to 9).
4. Floating Point Precision: When using double, $0.1 + 0.2$ might be $0.30000000000004$. Use a small EPS = 1e-9 for comparisons.

If you are stuck for 10 minutes:

• Read the problem AGAIN. Did you miss a constraint?
• Check your Fast I/O.
• Check if you reset your variables for multiple test cases.

Take a piece of paper and a pen. Trace your variables for the sample input. Computers are fast, but paper is smart.