LeetCode: 202. Happy Number

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202. Happy Number

LeetCode - Happy Number

竟然可以使用"双指针"，类似判断链表中是否有环的思路！妙！

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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/**
 * Runtime: 1 ms, faster than 99.99% of Java online submissions for Happy Number.
 *
 * Memory Usage: 33.4 MB, less than 10.60% of Java online submissions for Happy Number.
 *
 * Copy from: https://leetcode.com/problems/happy-number/discuss/56917/My-solution-in-C(-O(1)-space-and-no-magic-math-property-involved-)[My solution in C( O(1) space and no magic math property involved ) - LeetCode Discuss]
 */
public boolean isHappy(int n) {
    int slow = n;
    int fast = n;
    do {
        slow = digitSquareSum(slow);
        fast = digitSquareSum(fast);
        fast = digitSquareSum(fast);
        if (fast == 1) {
            return true;
        }
    } while (slow != fast);
    return false;
}

public int digitSquareSum(int n) {
    int sum = 0;
    while (n != 0) {
        int temp = n % 10;
        sum += temp * temp;
        n /= 10;
    }
    return sum;
}