[LeetCode-Simple Question] 202. Happy Number

topic

This question can actually be transformed into whether there is a ring;
because if n = 1 is finally obtained, no matter how you continue to calculate, it will still be 1, which means that a ring appears at the end. Return true
if the ring is not finally formed at n = 1 It is formed under the circumstances, indicating that there is a cycle, and returns false directly.

There is a ring:

There is no ring:

Method 1: Hash

class Solution {
    
    
    public boolean isHappy(int n) {
    
    
    Set<Integer> set = new HashSet<>();
    while( n != 1  &&  !set.contains(n)){
    
    
            set.add(n);
            n = getisHappy(n);
        }
    return n==1;
    }

    public int getisHappy(int n){
    
    
        int res = 0;
        while(n>0){
    
    
            int temp = n%10;
            res+= temp*temp;
            n = n /10;
        }
        return res;
    }
}

Method 2: Fast and slow pointers

class Solution {
    
    
    public boolean isHappy(int n) {
    
    
    int slow = n;
    int fast = getisHappy(n);
    while( fast != 1  &&  slow != fast ){
    
    
           slow = getisHappy(slow);
           fast = getisHappy(getisHappy(fast));
        }
    return fast==1;
    }

    public int getisHappy(int n){
    
    
        int res = 0;
        while(n>0){
    
    
            int temp = n%10;
            res+= temp*temp;
            n = n /10;
        }
        return res;
    }
}