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
我一开始写了递归但不知道怎么处理陷入无限递归的情况，因为我以为会无限不循环，但其实一定会循环的，因为正整数一共就INTEGER.MAX_INT个，所以一定会循环。
用Set做：
public boolean isHappy(int n) {
Set<Integer> inLoop = new HashSet<Integer>();
int squareSum,remain;
while (inLoop.add(n)) {
squareSum = 0;
while (n > 0) {
remain = n%10;
squareSum += remain*remain;
n /= 10;
}
if (squareSum == 1)
return true;
else
n = squareSum;
}
return false;
}
不用Set做：
public boolean isHappy(int n) {
if (n <= 0) return false;
if (n == 1) return true;
int slow = calc(n);
int fast = calc(n);
fast = calc(fast);
while (slow != fast) {
slow = calc(slow);
fast = calc(fast);
fast = calc(fast);
}
return slow == 1;
}
private int calc(int n) {
int sum = 0;
while (n > 0) {
int remain = n % 10;
sum += remain * remain;
n /= 10;
}
return sum;
}
