Happy Number

If n becomes 1 after repeatedly replacing it with the sum of squares of its digits, then the initial n is a happy number.

This problem tests your ability to detect cycles in results. We use the fast-slow pointer technique from cycle detection to see if n creates a loop after multiple iterations.

Floyd Cycle Detection

class Solution {
public:
    bool isHappy(int n) {
        int fast, slow;
        fast = sum(n);
        slow = n;
        while(fast!=slow) {
            slow = sum(slow); // slow moves one step
            fast = sum(fast); // fast moves two steps
            fast = sum(fast);
        }
        return slow == 1;
    }
    
    int sum(int n) {
        int s=0;
        int tmp;
        while(n) {
            tmp = n%10;
            s += tmp*tmp;
            n/=10;
        }
        return s;
    }
};

Gray Code Implement strStr