Leetcode 题目解析之 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

1. 使用set
   对每一个计算的结果进行计算，如果是1则是Happy Number；不是1且在set中出现过，则不是返回false；否则将结果加入set。
2. 利用性质，特殊数字“4”
   在某一时刻，出现了4的Number肯定不Happy。

    public boolean isHappy(int n) {
        if (n < 1) {
            return false;
        }
        if (n == 1) {
            return true;
        }
        Set<Integer> set = new HashSet<Integer>();
        set.add(n);
        while (true) {
            int s = 0;
            while (n > 0) {
                s += (n % 10) * (n % 10);
                n /= 10;
            }
            System.out.println(s);
            if (s == 1) {
                return true;
            } else if (set.contains(s)) {
                return false;
            }
            set.add(s);
            n = s;
        }
    }

    public boolean isHappy2(int n) {
        if (n < 1) {
            return false;
        }
        if (n == 1) {
            return true;
        } else if (n == 4) {
            return false;
        }
        int s = 0;
        while (n > 0) {
            s += (n % 10) * (n % 10);
            n /= 10;
        }
        return isHappy2(s);
    }