202. Happy Number
Tags: ‘Hash Table’, ‘Math’
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
// Assume either there is a loop or go back to 1 (not going infinite)
public boolean isHappy(int n) {
Set<Integer> set = new HashSet<>();
int squareSum = 0, remain = 0;
while (set.add(n)) {
squareSum = 0;
while (n > 0) {
remain = n % 10;
squareSum += remain * remain;
n /= 10;
}
if (squareSum == 1) return true;
n = squareSum;
}
return false;
}
//Floyd Cycle detection algorithm
public boolean isHappy(int n) {
int slow = n, fast = n;
do {
slow = squareSum(slow);
fast = squareSum(fast);
fast = squareSum(fast);
if (fast == 1) return true;
} while (slow != fast)
return 0;
}