Leetcode: 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

hash table检查是否重复出现，可以查表避免乘法运算。

class Solution {public:    bool isHappy(int n) {        if (n <= 0) {            return false;        }                int squareTable[] = {0, 1, 4, 9, 16, 25, 36, 49, 64, 81};        set<int> appearedNum;        while (appearedNum.find(n) == appearedNum.end()) {            appearedNum.insert(n);            int result = 0;            while (n != 0) {                int lowestDigit = n % 10;                n /= 10;                result += squareTable[lowestDigit];            }            n = result;        }                return (n == 1);    }};