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

我的解法：每次计算出和，判断是否为1，不为1则判断是否在set里（在set里，则跟之前的重复，若重复了，那么永远不能为1了，返回false），

class Solution {

    public boolean isHappy(int n) {
        Set<Integer> set=new HashSet<Integer>();
int sum=n;
while (true) {
n=sum;
sum = 0;
while (n > 0) {
sum += Math.pow(n % 10, 2);
n = n / 10;
}
if(sum==1)
return true;
else
{
if(set.contains(sum))
return false;
else
set.add(sum);
}
}
    }

}

不占用额外空间的c++算法，fast和slow分别作为指针，fast每次走两步，slow只走一步，等到fast追上slow时，判断slow是否为1，用时间换空间

作者， Freezen

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int digitSquareSum(int n) {
    int sum = 0, tmp;
    while (n) {
        tmp = n % 10;
        sum += tmp * tmp;
        n /= 10;
    }
    return sum;
}

bool isHappy(int n) {
    int slow, fast;
    slow = fast = n;
    do {
        slow = digitSquareSum(slow);
        fast = digitSquareSum(fast);
        fast = digitSquareSum(fast);
    } while(slow != fast);
    if (slow == 1) return 1;
    else return 0;
}