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

这道题挺简单的，就按照它的逻辑写就好了。

public boolean isHappy(int n) {HashSet<Integer> set=new HashSet<Integer>();while(n!=1&&!set.contains(n)){set.add(n);int sum=0;while(n>0){int yu=n%10;n=n/10;sum+=yu*yu;}n=sum;}if(n==1){return true;}else{return false;}}

大神表示：看到大多数人都使用hashset来记录值。而实际上，我们可以使用 Floyd Cycle detection 算法，使用在 Linked List Cycle 找环的入口的方法。既然之后有循环，我们可以看做后面有环。我们只需要看这个环的入口值是不是1。

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;}