202-快乐数

LeetCode 202. Happy Number

LeetCode

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, 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:
$1^2 + 9^2 = 82$
$8^2 + 2^2 = 68$
$6^2 + 8^2 = 100$
$1^2 + 0^2 + 0^2 = 1$

Example 2:

Input: n = 2
Output: false

method: 哈希表

记录每次平方和结果，如果出现循环返回false

int squareSum(int x) {
    int sum = 0;
    while (x) {
        sum += (x % 10) * (x % 10);
        x /= 10;
    }
    return sum;
}

bool isHappy(int n) {
    unordered_set<int> hash;
    while (1) {
        n = squareSum(n);
        if (n == 1) return true;
        if (hash.find(n) != hash.end()) return false;
        else hash.insert(n);
    }
    return false;
}