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. 快乐数的具体解释参考这里
解题思路
由上文的资料可得，不是快乐数的数称为不快乐数（unhappy number），所有不快乐数的数位平方和计算，最後都会进入 4 → 16 → 37 → 58 → 89 → 145 → 42 → 20 → 4 的循环中。 那么，可以用hashset保存这些值，遇到直接返回false即可。
代码语言：javascript复制public boolean isHappy(int n) {
if (n == 0) return false;
if (n == 1) return true;
int sum = 0;
HashSet<Integer> set = new HashSet<>();
set.add(4);
set.add(16);
set.add(37);
set.add(58);
set.add(89);
set.add(145);
set.add(42);
set.add(20);
while (n != 0) {
sum = (n % 10) * (n % 10);
n /= 10;
}
return !set.contains(sum) && isHappy(sum);
}
