202 Happy Number
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)
class Solution {
public boolean isHappy(int n) {
int fast = n, slow = n;
do {
slow = getNextNumber(slow);
fast = getNextNumber(getNextNumber(fast));
if (fast == 1) {
return true;
}
} while (slow != fast);
return false;
}
public int getNextNumber(int num) {
int sum = 0;
while (num > 0) {
sum += (num % 10) * (num % 10);
num /= 10;
}
return sum;
}
}
3. Time & Space Complexity
Two Pointers: 时间复杂度不详,空间复杂度O(1)
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