[leetcode] 202. Happy Number

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
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 =
1

这道题是确定一个数字是否是happy number，题目难度为easy。

按照题目规则，很自然会使用Hash Table，将每次计算的结果存入Hash Table，直到结果是1表明数字是happy number，在这之前如果遇到相同的结果则不是happy number。具体代码：

class Solution {
public:
bool isHappy(int n) {
unordered_set<int> num;
while(true) {
int sum = 0;
while(n) {
sum += (n%10)*(n%10);
n /= 10;
}
n = sum;
if(sum == 1) return true;
if(num.find(sum) != num.end()) return false;
else num.insert(sum);
}
}
};

另外还有数学的方法，感兴趣的同学可以自己证明。