[LeetCode][JavaScript]Happy Number

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

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

https://leetcode.com/problems/happy-number/

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累加每一位数字的平方数，如果等于1就是happy number，否则递归去找，遇到死循环说明不是。

常规思路是开个哈希表，记录已经出现的数，一遇到重复就返回false。

 

使用带哈希表的代码，把个位数打个表，发现只有1和7是happy number，其他都不是。

那么为了去掉哈希表，我直接写死了1-9的结果，因为happy number的运算很多中间结果都是个位数，遇到个位数就可以不用做下去了。

 

/**
 * @param {number} n
 * @return {boolean}
 */
var isHappy = function(n) {
    if(n === 1 || n === 7) return true;
    if(n < 10) return false;
    n = n.toString();
    var nextNum = 0, tmp;
    for(var i = 0; i < n.length; i++){
        tmp = Math.floor(n[i]);
        nextNum += tmp * tmp;
    }
    return isHappy(nextNum);
};