SomePointsduringCPP.txt

===========================
CORE NOTES (DSA + C++)
===========================

1) -1 in C++ indicates that an element does not exist.
   Since array indexing always starts from 0, an index can never be -1.

2) When a question mentions "NO EXTRA SPACE",
   always try to use the TWO POINTER APPROACH.

3) Arrays created additionally to store values of another array
   are called AUXILIARY ARRAYS.

4) In a 2D matrix, it is mandatory to specify the number of columns
   while passing it to a user-defined function OR use double pointer.

   Examples:
   void spiralMatrix(int **mat, int n, int m)
   void spiralMatrix(int mat[][100], int n, int m)

5) Difference between char array and string:
   - Char array is a simple array of characters.
   - String is an object of the string class.
   - Strings are dynamic in size.
   - Char arrays are fixed in size.

6) In OOP, if no access modifier is specified inside a class,
   the default access modifier is PRIVATE.

7) To decide whether recursion is required:
   Recursion is usually associated with exponential time complexity,
   so use it only when the problem structure demands it.

8) Sudoku always has a FIXED size of 9x9.

9) STL containers are passed by VALUE by default.
   To modify the original container, pass it by reference using &.

10) template <class T>
    Template class allows using any data type for data structures
    or STL-like functionality.

11) STL containers usually do not change original data
    unless passed by reference (&).

12) Problems involving maximum/minimum (other than simple arrays/strings),
    such as maximum subarray, substring, subsequence, max-sum, min-sum,
    often indicate the use of GREEDY ALGORITHMS.

13) Take or not take decision problems:
    - 0/1 Knapsack -> TV, Fridge, Washing Machine.
    - Fractional Knapsack -> Sugar, Salt, Oil (quantity matters).

14) In Binary Trees:
    - Traversal is possible only from Parent to Child.
    - Backtracking is possible ONLY using recursion.

15) Return conventions:
    - If return type is int and nothing exists -> return -1.
    - If return type is Node* and nothing exists -> return NULL.

===========================
TWINKLING LOGIC PARTS
===========================

1) Array Right Rotation:
   Reverse loop (n-1 to 1)
   arr[i] = arr[i-1]

2) Array Left Rotation:
   Straight loop (0 to n-2)
   arr[i] = arr[i+1]

3) Matrix 180 Degree Rotation:
   Swap rows n/2 times, then reverse each row.

4) Matrix 90 Degree Clockwise:
   Transpose matrix, then reverse each row
   OR after transpose, swap columns n/2 times.

5) Matrix 90 Degree Anti-Clockwise:
   Transpose matrix, then swap rows n/2 times.

===========================
ON HOLD TOPICS
===========================

1) Common letter sequences in string.
2) Two strings are rotation of each other.

===========================
STL SYNTAX TO REMEMBER
===========================

1) Binary Search:
   if (binary_search(arr, arr + n, findElement))
       return 1;
   else
       return 0;

2) Vector:
   vector<int> v;
   vector<int> v2(5);
   vector<int> v3(5, 1);

3) Set:
   set<int> st;
   st.insert(51);
   st.size();
   st.erase(51);

4) Multiset:
   multiset<int> mst;
   mst.insert(12);

In vectors Sorting is also same as - 

// Ascending sort
sort(arr.begin(), arr.end());

// Descending sort
sort(arr.begin(), arr.end(), greater<int>());

// Reverse vector
reverse(arr.begin(), arr.end());


===========================
TERNARY OPERATOR SYNTAX
===========================

int age = 16;
bool isAdult = (age > 18)
               ? (cout << "Yes Mature" << endl, true)
               : (cout << "Immature" << endl, false);
cout << isAdult;

===========================
END OF NOTES
===========================
*/

int main()
{
    // This file is intentionally kept logic-free.
    // Used purely as a structured DSA + C++ notes reference.
    return 0;
}