202 Happy Number

# 1. Question

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: 19 is a happy number
• 1 ^ 2 + 9^2 = 82
• 8 ^ 2 + 2 ^ 2 = 68
• 6 ^ 2 + 8 ^ 2 = 100
• 1 ^ 2 + 0 ^ 2 + 0 ^ 2 = 1

# 2. Implementation

(1) Two Pointers (Cycle Detection)
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class Solution {
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public boolean isHappy(int n) {
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int fast = n, slow = n;
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do {
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slow = getNextNumber(slow);
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fast = getNextNumber(getNextNumber(fast));
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if (fast == 1) {
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return true;
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}
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} while (slow != fast);
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return false;
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}
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public int getNextNumber(int num) {
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int sum = 0;
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while (num > 0) {
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sum += (num % 10) * (num % 10);
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num /= 10;
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}
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return sum;
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}
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}
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# 3. Time & Space Complexity

Two Pointers: 时间复杂度不详，空间复杂度O(1)