LeetCode-202.Happy Number

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
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 =
1

比较暴力的解法：

public class Solution
{
public bool IsHappy(int n)
{
HashSet<int> list = new HashSet<int>();
list.Add(n);
Stack<int> nums = new Stack<int>();
Fun(n, nums);
int sum;
while (true)
{
sum=0;
while (nums.Count > 0)
{
n = nums.Pop();
sum += n * n;
}
if (sum == 1)
return true;
if (!list.Add(sum))
return false;
Fun(sum, nums);
}
}

private void Fun(int n, Stack<int> nums)
{
string s = n.ToString();
for (int i = 0; i < s.Length; i++)
{
nums.Push(Convert.ToInt16(s[i]-'0'));
}
}
}

优化：

public bool IsHappy(int n)
{
HashSet<int> list = new HashSet<int>();
list.Add(n);
int sum,num;
while (true)
{
sum = 0;
while (n > 0)
{
num = n % 10;
sum += num * num;
n /= 10;
}
if (sum == 1)
return true;
if (!list.Add(sum))
return false;
n = sum;
}
}

Floyd Cycle detection algorithm周期检测算法

public class Solution
{
public bool IsHappy(int n)
{
int slow = n, fast = n;
while (true)
{
slow = Fun(slow);
fast = Fun(fast);
fast = Fun(fast);
if (slow == fast)
break;
}
return slow == 1;
}
private int Fun(int n)
{
int num,sum = 0;
while (n > 0)
{
num = n % 10;
sum += num * num;
n /= 10;
}
return sum;
}
}