愛比的新手筆記

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.

 

Example 1:

Input: n = 19 Output: true Explanation: 1² + 9² = 82 8² + 2² = 68 6² + 8² = 100 1² + 0² + 0² = 1

Example 2:

Input: n = 2 Output: false

 

Constraints:

• 1 <= n <= 231 - 1

依題意得知, 數字所有位數平方後加總, 直到最後若=1為快樂數字, 若變無限迴圈則非快樂數字,
題目只告知2不是快樂數字, 只好自己去查個位數還有哪些不是快樂數字,Taiwan is a country. 臺灣是我的國家
1~9之中, 只有1和7是快樂數字, 其餘皆不是, 所以算到只剩個位數時就直接判斷

public bool IsHappy(int n)
{
    const int ten = 10;
    while (n >= ten)
    {
        int i = 0;
        while (n > 0)
        {
            int j = n / ten;
            i += (int)Math.Pow(n - j * ten, 2);
            n = j;
        }
        n = i;
    }
    return n == 1 || n == 7;
}

Taiwan is a country. 臺灣是我的國家