202.Happy Number

Tags: Easy Hash Table Math

Links: https://leetcode.com/problems/happy-number/

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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, 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.

Return True if n is a happy number, and False if not.

Example:

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

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class Solution {
public:
    bool isHappy(int n) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        unordered_set<int> us;
        while (us.find(n) == us.end()) {
            us.emplace(n);
            if (n == 1) return true;
            n = calculate(n);
        }

        return false;
    }

    int calculate(int n)
    {
        int sum = 0;
        while (n) {
            int mode = n % 10;
            sum += mode * mode;
            n /= 10;
        }
        return sum;
    }
};

因为题目指明可能存在循环，并且循环的点不为1，那么就用一个unordered_set来解决。

另外这道题存在快慢指针的解法：

class Solution {
public:
    bool isHappy(int n) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        int slow = n, fast = n;
        while (true) {
            slow = calculate(slow);
            fast = calculate(fast);
            fast = calculate(fast);
            if (slow == fast) break;
        }

        return slow == 1;
    }

    int calculate(int n)
    {
        int sum = 0;
        while (n) {
            int mode = n % 10;
            sum += mode * mode;
            n /= 10;
        }
        return sum;
    }
};