LeetCode题解——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

最重要的怎样跳出循环，两种方法：1.参考判断链表是否是一个循环链表的方法：fast和slow指针； 2.利用hashset,保存计算过程，若出现重复的数，则不是happy number。
方法一：4ms

class Solution {
public:
int digitSquareSum(int n) {
int sum = 0, tmp;
while (n) {
tmp = n % 10;
sum += tmp * tmp;
n /= 10;
}
return sum;
}

bool isHappy(int n) {
int slow, fast;
slow = fast = n;
do {
slow = digitSquareSum(slow);
fast = digitSquareSum(fast);
fast = digitSquareSum(fast);
} while(slow != fast);
if (slow == 1) return 1;
else return 0;
}
};

方法2:4ms

class Solution {
public:
bool isHappy(int n) {
set<int> looktable;
while(looktable.find(n)==looktable.end()){
looktable.insert(n);
n = digitSquareSum(n);
if(n==1) return true;
}
return false;
}
int digitSquareSum(int n){
int sum =0 ;
while(n){
sum+=pow((n%10),2);
n=n/10;
}
return sum;
}
};