[LeetCode] 202. Happy Number

Write an algorithm to determine if a number n 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.

Return True if n is a happy number, and False if not.

寻找快乐数字。快乐数字的定义是如果每一位上的数字的平方相加最后能成为1，那么这个数字就是快乐数字。

思路就是按照其规则做运算，如果最后是false，中间一定是有死循环，此处可以用hashset记录出现过的和。如果最后能算成1，就是true。

时间O(1) - 因为稍微算几次就会出来，不会因着input变大而变大

空间O(n) - 用了一个hashmap

Java实现

class Solution {
    public boolean isHappy(int n) {
        HashSet<Integer> set = new HashSet<>();
        while (n != 1) {
            int sum = 0;
            while (n > 0) {
                sum += (n % 10) * (n % 10);
                n /= 10;
            }
            if (set.contains(sum)) {
                return false;
            } else {
                set.add(sum);
            }
            n = sum;
        }
        return true;
    }
}

JavaScript实现

/**
 * @param {number} n
 * @return {boolean}
 */
var isHappy = function (n) {
    let set = new Set();
    while (n !== 1) {
        let sum = 0;
        while (n > 0) {
            sum += (n % 10) * (n % 10);
            n = parseInt(n / 10);
        }
        if (set.has(sum)) {
            return false;
        } else {
            set.add(sum);
            n = sum;
        }
    }
    return true;
};

LeetCode 题目总结