## 202. 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.
• 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.

Return `true` if `n` is a happy number, and `false` if not.

Example 1:

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

Example 2:

```Input: n = 2
Output: false
```

Constraints:

• `1 <= n <= 231 - 1`

## Rust Solution

``````struct Solution;

impl Solution {
fn digit_square_sum(mut x: i32) -> i32 {
let mut sum = 0;
while x > 0 {
let tmp = x % 10;
sum += tmp * tmp;
x /= 10;
}
sum
}

fn is_happy(n: i32) -> bool {
let mut slow = n;
let mut fast = n;
loop {
slow = Self::digit_square_sum(slow);
fast = Self::digit_square_sum(fast);
fast = Self::digit_square_sum(fast);
if slow == fast {
break;
}
}
slow == 1
}
}

#[test]
fn test() {
assert_eq!(Solution::is_happy(19), true);
}
``````

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