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;    }};