LeetCode 202. Happy Number

考点	难度
Hash Table	Easy

题目

Write an algorithm to determine if a number n 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.
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.
Return true if n is a happy number, and false if not.

思路

@miyamura的答案解释得很清楚，遇到和cycle有关的问题都要用Floyd’s cycle-finding algorithm（两个pointers，一个fast一个slow）。如果有cycle，fast pointer会和slow pointer相遇（指向同一个node）。如果没有cycle，fast pointer会到达loop的终点（null）。
根据题目描述，如果n不是happy number则会出现cycle，所以需要用到Floyd’s cycle-finding algorithm。

答案

public int compute(int n){
        int s = 0;
        while(n > 0){
            s += (n % 10) * (n % 10);
            n = n / 10;
        }
        return s;
    }
    
    public boolean isHappy(int n) {
        int slow = n;
        int fast = compute(n);
        while (fast != 1 && slow != fast){ // happy number和有cycle时的停止条件
            slow = compute(slow);
            fast = compute(compute(fast));
        }
        return fast == 1;
    }