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contents/algorithms/math.md

@@ -3,6 +3,33 @@ id: math
 title: Math
 ---
 
+## Notes
+
+If code involves division or modulo, remember to check for division or modulo by 0 case.
+
+When a question involves "a multiple of a number", perhaps modulo might be useful.
+
+Check for and handle overflow/underflow if you are using a typed language like Java and C++. At the very least, mention that overflow/underflow is possible and ask whether you need to handle it.
+
+Consider negative numbers and floating point numbers. This may sound obvious, but under interview pressure, many obvious cases go unnoticed.
+
+If the question asks to implement an operator such as power, squareroot or division and want it to be faster than O(n), binary search is usually the approach to go.
+
+#### Some common formulas:
+
+- Sum of 1 to N = (n+1) \* n/2
+- Sum of GP = 2<sup>0</sup> + 2<sup>1</sup> + 2<sup>2</sup> + 2<sup>3</sup> + ... 2<sup>n</sup> = 2<sup>n+1</sup> - 1
+- Permutations of N = N! / (N-K)!
+- Combinations of N = N! / (K! \* (N-K)!)
+
+## Recommended Leetcode questions
+
+- [Pow(x, n)](https://leetcode.com/problems/powx-n/)
+- [Sqrt(x)](https://leetcode.com/problems/sqrtx/)
+- [Integer to English Words](https://leetcode.com/problems/integer-to-english-words/)
+
+## More questions
+
 - Create a square root function.
 - Given a string such as "123" or "67", write a function to output the number represented by the string without using casting.
 - Make a program that can print out the text form of numbers from 1 - 1000 (ex. 20 is "twenty", 105 is "one hundred and five").