Precompute all the solutions in C++


  • 2
    F

    Since positive int has no more than 11 digits, its digits' square sum falls in the range from 1 ~ 9911
    Welcome to visit all the answers on: https://github.com/fanfank/leetcode

    //time: O(1)
    //space: O(1) <- O(891)
    class Solution {
    private:
        //0: uninitialized
        //1: happy num
        //2: not happy num
        //3: being set
        int answer[9*9*11];                                                                                                                                                                                                                      
    
    public:
        Solution() {
            memset(answer, 0, sizeof(answer));
            for (int i = 1; i < 9 * 9 * 11; ++i) {
                judgeHappy(i);
            }
        }
    
        int judgeHappy(int num) {
            if (answer[num] == 1 || answer[num] == 2) {
                return answer[num];
            } else if (answer[num] == 3) {
                return 2;
            } else if (num == 1) {
                return answer[num] = 1;
            }
    
            answer[num] = 3;
    
            return answer[num] = judgeHappy(digitSquareSum(num));
        }   
    
        int digitSquareSum(int num) {
            int res = 0, digit = 0;
            while (num != 0) {
                digit = num % 10; 
                res += digit * digit;
                num /= 10; 
            }   
            return res;
        }
    
        bool isHappy(int n) {
            return answer[digitSquareSum(n)] == 1 ? true : false;
        }   
    };

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