# 202-happy-number

## Question

https://leetcode.com/problems/happy-number/description/

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
``````

## Thought Process

1. Brute Force - Set
1. Use set to record the squared sum through the process
2. If we have seen this number before, we know there is loop, and we cannot reach to 1
2. asd

## Solution

``````class Solution {
public boolean isHappy(int n) {
Set<Integer> set = new HashSet<>();
int res = 0;
if (n == 1) return true;
while (n > 0) {
int d = n % 10;
res += d * d;
n /= 10;
}
n = res;
res = 0;
}
return false;
}
}
``````
``````class Solution {
public boolean isHappy(int n) {
int slow = n, fast = n;
do {
slow = cal(slow);
fast = cal(cal(fast));
} while (slow != fast);
return slow == 1;
}

private int cal(int n) {
int res = 0;
while (n > 0) {
int d = n % 10;
res += d * d;
n /= 10;
}
return res;
}
}
``````