leetcode [#202]

目录

• 1. 题目
• 2. 解决方案
• 3. 注意事项

题目

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

---

解决方案

public class Solution {
    public boolean isHappy(int n) {
        StringBuilder builder = new StringBuilder(String.valueOf(n));
        int sum = 0;
        for(int i = 0; i < builder.length(); i++) sum += (builder.charAt(i) - '0') * (builder.charAt(i) - '0');
        HashSet set = new HashSet();
        set.add(sum);
        while(sum != 1){
            StringBuilder bu = new StringBuilder(String.valueOf(sum));
            sum = 0;
            for(int i = 0; i < bu.length(); i++) sum += (bu.charAt(i) - '0') * (bu.charAt(i) - '0');
            if(!set.add(sum)) return false;
        }
        return true;
    }
}

---

注意事项

1. 基本思路是：使用一个HashSet来存储每次求得的平方和，在和不为1的前提条件下，如果add方法失败，说明出现了重复值，即说明会有循环，不满足happy number条件，返回false。