leetcode：202. 快乐数

题目来源

• leetcode：202. 快乐数

题目描述

题目解析

模拟

class Solution {
    
public:
    bool isHappy(int n) {
    
        std::set<int> set;
        set.insert(n);
        while (n != 1){
    
            int sum = 0;
            while (n != 0){
      // 2
                sum += std::pow(n % 10, 2);  // ans = 4
                n = n / 10;  // 0
            }
            if(set.count(sum)){
    
                return false;
            }
            set.insert(sum);
            n = sum;
        }
        return true;
    }
};

快慢指针

怎么检测环呢？快慢指针

class Solution {
    
    int getNext(int n){
    
        int sum = 0;
        while(n > 0)
        {
    
            int bit = n % 10;
            sum += bit * bit;
            n = n / 10;
        }
        return sum;


    }

public:
    bool isHappy(int n) {
    
        int  slow = n, fast = n;
        do {
    
            slow = getNext(slow);
            fast = getNext(getNext(fast));
        }while (fast != slow);
        return fast == 1;
    }
};