Happy number

Hash Table Math Two Pointers

59.3% acceptance

Jan 12, 2026

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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.

Solution

Rust

Time O(2^n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn is_happy(n: i32) -> bool {
    let mut slow = n;
    let mut fast = n;
    
    loop {
      slow = Self::get_next(slow);
      fast = Self::get_next(Self::get_next(fast));
      if fast == 1 { return true; }
      if slow == fast { return false; }
    }
  }
  
  fn get_next(mut n: i32) -> i32 {
    let mut sum = 0;
    while n > 0 {
      let digit = n % 10;
      sum += digit * digit;
      n /= 10;
    }
    sum
  }
}