LeetCode 30天挑戰 Day-2

LeetCode 30 days Challenge - Day 2

本系列將對LeetCode新推出的30天算法挑戰進行總結記錄，旨在記錄學習成果、方便未來查閱，同時望爲廣大網友提供幫助。

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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:

Input: 19
Output: true
Explanation: 
1^2 + 9^2 = 82
8^2 + 2^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1

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題目要求分析：給定一個整數，將該整數每一位平方加和得到新的整數，重複直到整數爲1或出現循環。

解法：

快慢指針：遇到出現循環情況的題目，可以考慮快慢指針的方法：

• 快指針：一次走2步（在本題中即進行兩次運算）。
• 慢指針：一次走1步（進行一次運算）。

在若干次迭代後，如果存在循環，快慢指針會指向同一值，此時退出循環即可。

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Solution

class Solution {
public:
    int get_pow(int n) {
        int tmp = 0;
        while (n) {
            tmp += (n % 10) * (n % 10);
            n /= 10;
        }
        return tmp;
    }
    bool isHappy(int n) {
        if (n == 1) return true;
        int slow = n, fast = n;
        do {
            slow = get_pow(slow);
            fast = get_pow(fast);
            fast = get_pow(fast);
        } while (slow != fast);
        return (slow == 1);
    }
};

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傳送門：Happy Number

2020/4 Karl