for fast I/O:
ios_base::sync_with_stdio(false);
cin.tie(NULL);

Greedy algorithms : at every step, we make the choice that looks best at that moment and we get the optimal solution for the entire problem.

Backtracking : Go ahead with the approach, check if satisfied. If not, come back. Start with another alternative.

Skip list: an advanced data structure that introduces multi-layers in linked lists for linear traversal and complexity is reduced from O(n) (for simple linked list) to O(sqrt(n)). It can be apllied for sorted linked lists.

Binary search for special cases: {O(log n)} can be used to search for element in the sorted array that has been rotated once (pivot is kept as the element whose next element is smaller)

for substrings - use concept of sliding window

for non-repeating sequences - use unordered (or hash) set or simply set

Recursion

for assigning some default value to a 2d vector

    vector<vector<int>> v(r, vector<int>(c,default_value));

for converting int to string and vice-versa

string sp = to_string(m);

int ans = stoi(s);

Set Bit Count in Java

	// Convert integer number to binary format 

	System.out.println(Integer.toBinaryString(a)); 

	// to print number of 1's in the number a 

	System.out.println(Integer.bitCount(a));