202.Happy-Number

202. Happy Number

题目地址

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

题目描述

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: 

Input: 19
Output: true
Explanation: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

代码

Approach #1 HashSet

class Solution {
  public boolean isHappy(int n) {
        Set<Integer> seen = new HashSet<>();
    while (n != 1 && !seen.contains(n)) {
      seen.add(n);
      n = getNext(n);
    }
    return n == 1;
  }

  private int getNext(int n) {
    int totalSum = 0;
    while (n > 0) {
      int d = n % 10;
      n = n / 10;
      totalSum += d * d;
    }
    return totalSum;
  }
}

Approach #2 Floyd's Cycle-Finding Algorithm

class Solution {
  public boolean isHappy(int n) {
    int slowRunner = n;
    int fastRunner = getNext(n);
    while (fastRunner != 1 && slowRunner != fastRunner) {
      slowRunner = getNext(slowRunner);
      fastRunner = getNext(getNext(fastRunner));
    }
    return fastRunner == 1;
  }

  private int getNext(int n) {
    int totalSum = 0;
    while (n > 0) {
      int d = n % 10;
      n = n  / 10;
      totalSum += d * d;
    }
    return totalSum;
  }
}