Question

Formatted question description: https://leetcode.ca/all/202.html

 202	Happy Number

 Write an algorithm to determine if a number 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.

 Example: 19 is a happy number

 1^2 + 9^2 = 82
 8^2 + 2^2 = 68
 6^2 + 8^2 = 100
 1^2 + 0^2 + 0^2 = 1

Algorithm

The example 19 given in the title is a happy number, so let’s look at a situation that is not a happy number. For example, the number 11 has the following calculation process:

1^2 + 1^2 = 2 2^2 = 4 4^2 = 16 1^2 + 6^2 = 37 3^2 + 7^2 = 58 5^2 + 8^2 = 89 8^2 + 9^2 = 145 1^2 + 4^2 + 5^2 = 42 4^2 + 2^2 = 20 2^2 + 0^2 = 4

At the end of the calculation, the number 4 appears again, then the following numbers will repeat the previous order. This cycle does not contain 1, then the number 11 is not a happy number. After discovering the pattern, we can consider how to use code it.

Use HashSet to record all the numbers that have appeared, and then every time a new number appears, look for it in HashSet to see if it exists,

  • if it does not exist, add it to the table,
  • if it exists, jump out of the loop, and determine whether the number is 1,
    • if it is 1. Return true,
    • return false if not 1

Code

Java

    class Solution {
        public boolean isHappy(int n) {

            HashSet<Integer> set = new HashSet<Integer>();

            while (!set.contains(n)) {
                set.add(n);

                n = getSum(n);

                if (n == 1) {
                    return true;
                }
            }

            return false;
        }

        public int getSum(int current) {
            int sum = 0;

            while (current > 0) {
                sum += Math.pow(current % 10, 2);
                current /= 10;
            }

            return sum;
        }
    }

    class Solution_iteration {
        public boolean isHappy(int n) {
            HashSet<Integer> st = new HashSet<Integer>();
            while (n != 1) {
                int sum = 0;
                while (n > 0) {
                    sum += (n % 10) * (n % 10);
                    n /= 10;
                }
                n = sum;
                if (st.contains(n)) {
                    break;
                }
                st.add(n);
            }

            return n == 1;
        }
    }

    // if find 4 then unhappy
    class Solution_math {
        public boolean isHappy(int n) {
            while (n != 1 && n != 4) {
                int sum = 0;
                while (n > 0) {
                    sum += (n % 10) * (n % 10);
                    n /= 10;
                }
                n = sum;
            }
            return n == 1;

        }
    }
}

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