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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