202. Happy Number

Problem:

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:

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

思路：

Solution (C++):

bool isHappy(int n) {
    vector<int> res = digit(n);
    int ans = square_sum(res);
    if (ans == 1) return true;
    set<int> s;
    s.insert(ans);
    while (ans != 1) {
        res = digit(ans);
        ans = square_sum(res);
        if (ans == 1) return true;
        if (s.find(ans) != s.end()) return false;
        else s.insert(ans);
    }
    return false;
}

vector<int> digit(int n) {
    vector<int> dig;
    if (n == 0) { dig.push_back(0); return dig; }
    while (n) {
        dig.push_back(n%10);
        n /= 10;
    }
    return dig;
}

int square_sum(vector<int> &v) {
    int sum = 0;
    for (auto num: v) sum += num * num;
    return sum;
}

性能：

Runtime: 8 ms  Memory Usage: 6.8 MB

思路：

Solution (C++):

性能：

Runtime: ms  Memory Usage: MB