LeetCode之“数学”：Happy Number

　　题目链接

　　题目要求：

　　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

• 1^2 + 9^2 = 82
• 8^2 + 2^2 = 68
• 6^2 + 8^2 = 100
• 1^2 + 0^2 + 0^2 = 1

　　Credits:
　　Special thanks to @mithmatt and @ts for adding this problem and creating all test cases.

　　这道题比较需要注意的一点是要在合适的时候跳出死循环。程序如下：

class Solution {
public:
    bool isHappy(int n) {
        vector<int> vec;
        vec.push_back(n);
        while(n!= 1)
        {
            int sum = 0;
            int tmpN = n;
            while(tmpN != 0)
            {
                sum += (tmpN % 10) * (tmpN % 10);
                tmpN /= 10;
            }
            n = sum;
            
            if(find(vec.begin(), vec.end(), n) != vec.end())
                break;
            else
                vec.push_back(n);
        }
        
        return n == 1;
    }
};

　　改为用哈希表来实现的程序如下：

class Solution {
public:
    bool isHappy(int n) {
        unordered_map<int, int> hashMap;
        hashMap[n] = n;
        while(n != 1)
        {
            int sum = 0;
            int tmpN = n;
            while(tmpN != 0)
            {
                sum += (tmpN % 10) * (tmpN % 10);
                tmpN /= 10;
            }
            n = sum;
            
            if(hashMap.find(n) != hashMap.end())
                break;
            else
                hashMap[n] = n;
        }
        
        return n == 1;
    }
};