Why My Code Works Locally But Fails on Submission — Common Reasons

# Always test edge cases before submitting
def solve(n, arr):
    # Your solution here
    pass

# Read exactly as judge sends — bulk read is fastest
import sys

def main():
    input_data = sys.stdin.read().split()
    idx = 0

    t = int(input_data[idx]); idx += 1  # number of test cases
    for _ in range(t):
        n = int(input_data[idx]); idx += 1
        arr = [int(input_data[idx+i]) for i in range(n)]
        idx += n
        print(solve(n, arr))

# Edge case tests — always run these before submitting
print("=== Edge cases ===")
print(solve(1, [0]))           # single element zero
print(solve(1, [10**9]))       # max value
print(solve(0, []))            # empty input
print(solve(5, [1,1,1,1,1]))   # all same
print(solve(5, [-1,-2,-3,-4,-5]))  # all negative

main()  # comment out for submission

#include <bits/stdc++.h>
using namespace std;

// Fast I/O — required for large inputs
ios_base::sync_with_stdio(false);
cin.tie(NULL);

// Use long long to prevent overflow
long long n, m;
cin >> n >> m;

// Dangerous: n*n might overflow if n is large
// long long result = n * n;  // ❌ if n is int

// Safe: explicitly cast to long long before multiply
long long result = (long long)n * n;  // ✅

// Or declare n as long long from the start
long long a, b;
cin >> a >> b;
long long product = a * b;  // safe — both long long

name: CI
on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      # Pin exact Node version — match your local version
      - uses: actions/setup-node@v4
        with:
          node-version: '20.11.0'  # exact version, not just '20'
          cache: 'npm'

      # Verify env vars are present
      - name: Check required env vars
        run: |
          test -n "$DATABASE_URL" || (echo "Missing DATABASE_URL" && exit 1)
          test -n "$API_KEY" || (echo "Missing API_KEY" && exit 1)
        env:
          DATABASE_URL: ${{ secrets.DATABASE_URL }}
          API_KEY: ${{ secrets.API_KEY }}

      - run: npm ci
      - run: npm test

// Run these checks at app startup — fail fast with clear errors
const required = ['DATABASE_URL', 'API_KEY', 'SECRET_KEY'];
const missing = required.filter(k => !process.env[k]);
if (missing.length > 0) {
  throw new Error('Missing required env vars: ' + missing.join(', '));
}

// Use path.join for cross-platform file paths
const path = require('path');
const dataPath = path.join(__dirname, '..', 'data', 'config.json');
// Not: '../data/config.json' (works on Mac, may fail on Windows CI)

// Log runtime environment to catch version mismatches
console.log('Node version:', process.version);
console.log('Platform:', process.platform);
console.log('Environment:', process.env.NODE_ENV);

// Always use exact case in imports (for Linux servers)
// ✅ import Button from './Button'    (file is Button.tsx)
// ❌ import Button from './button'    (file is Button.tsx — fails on Linux)

# Use exact version tag, never :latest
FROM node:20.11.0-alpine3.19

WORKDIR /app

# Copy package files first (for layer caching)
COPY package*.json ./
RUN npm ci --only=production

COPY . .

# Build step
RUN npm run build

# Run as non-root user for security
USER node

EXPOSE 3000
CMD ["node", "server.js"]