LeetCode 202 Happy Number

题目：

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:

解法：

1. 利用set保存每次计算的结果，若出现1，则返回true，若出现已有元素，而没有出现1则说明会陷入循环，返回false

class Solution {
    public boolean isHappy(int n) {
        
        Set<Integer> set = new HashSet<>();
        int newn = 0;
        
        while(!set.contains(n))
        {
            set.add(n);
            newn = 0;
            while(n>0)
            {
                newn += (n%10)*(n%10);
                n /= 10;
            }
            if(newn==1) return true;
            
            n = newn;
        }
        
        return false;
        
    }
}

View Code

2. 参考了discussion的做法，经规律总结，当计算结果在10以内时，只有7和1才会是happy number

class Solution { public boolean isHappy(int n) { int newn = 0; while(n>9) { newn = 0; while(n>0) { newn += (n%10)*(n%10); n /= 10; } n = newn; } return n==7||n==1; } }

class Solution { public boolean isHappy(int n) { int slow = n; int fast = n; do{ slow = helper(slow); fast = helper(helper(fast)); if(slow==1||fast==1) return true; }while(slow!=fast); return false; } public int helper(int n) { int res = 0; while(n>0) { res += (n%10)*(n%10); n /= 10; } return res; } }

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