Description
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: 19 is a happy number
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Solution
很有意思的题啊，很纠结如何处理死循环，经点播发现可以使用一个HashSet做辅助，存储遇到过的数字。如果当前数字已经在HashSet中存在了，说明已经陷入死循环，就可以返回false了；
class Solution {
public boolean isHappy(int n) {
if (n == 1) return true;
Set<Integer> inLoop = new HashSet<>(); // nums already met
while (inLoop.add(n)) { // if already in set, it returns false
n = sumOfSquares(n);
if (n == 1) return true;
}
return false;
}
public int sumOfSquares(int n) {
int sum = 0;
while (n > 0) {
int digit = n % 10;
sum += digit * digit;
n /= 10;
}
return sum;
}
}
