Leetcode coding challenge notes

Question: Happy Number

Write an algorithm to determine if a number n is “happy”.

A happy number is a number defined by the following process: Starting with any positive integer, replace the number by the sum of the squares of its digits, and repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy numbers.

Return True if n is a happy number, and False if not.

Example:

​ Input: 19
​ Output: true
​ Explanation:
​ 12 + 92 = 82
​ 82 + 22 = 68
​ 62 + 82 = 100
​ 12 + 02 + 02 = 1

Notes

We can use % and / operations to get a lot of interesting things, such as each digit of an integer, or random number in a certain range.

For example, we know:

  1. 82 % 10 = 2;
  2. 82 / 10 = 8;

If we combine them and put them into a loop, we can get every digit in a very clever way.

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while(digits != 0) {
	digit = digits % 10;  
	digits = digits / 10;
}

Another example:

Assuming n and m are both integer, we know:

if m = n % 2, then 0 <= m <=1

if m = n % 10, then 0<= m <=9

if m = n % k, then 0<= m <= k-1

What if I want to get a random number in a certain range, what should we do?

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//from 0 to n-1
Random rand = new Random();
int m = Math.abs(rand.nextInt()) % n;

// from min to max
Random rand = new Random();
int m = rand.nextInt((max-min) + 1) + min