readme.md

Leetcode

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

Solution

1. 使用set集合来做.
2. 使用找规律来做 如果是非happy的数字, 那么一定会出现如果不是快乐数 最终会出现4 16 37 58 89 145 42 20循环
3. 判断有没有环的方式: 快慢指针.

一般判断有没有环的问题都使用快慢指针来解决

if 有环, 那么一定会在慢指针跑一圈, 快指针跑两圈的时候相等.
else 无环. 那么到头了都会停下来

那么检查最后停下来的点是不是在环的末尾就可以了.

package leetcode202_HappyNumber;

public class Solution3 {
  public boolean isHappy(int n) {

    int slowCursor = n;
    int fastCursor = n;
    do {
      slowCursor = getHappyResult(slowCursor);
      fastCursor = getHappyResult(fastCursor);
      fastCursor = getHappyResult(fastCursor);
    } while (slowCursor != fastCursor);

    return fastCursor == 1;
  }

  private int getHappyResult(int number) {
    int result = 0;

    for (int i = number; i != 0; i = i / 10) {
      result += Math.pow(i % 10, 2);
    }
    return result;
  }

  public static void main(String[] args) {
    System.out.println(new Solution3().isHappy(19));
  }
}