287. Find the Duplicate Number (Medium)
https://leetcode.com/problems/find-the-duplicate-number/
Given an array nums containing n + 1 integers where each integer is between 1 and n (inclusive), prove that at least one duplicate number must exist. Assume that there is only one duplicate number, find the duplicate one.
Example 1:
Input: [1,3,4,2,2]
Output: 2
Example 2:
Input: [3,1,3,4,2] Output: 3
Note:
- You must not modify the array (assume the array is read only).
- You must use only constant, O(1) extra space.
- Your runtime complexity should be less than O(n2).
- There is only one duplicate number in the array, but it could be repeated more than once.
Solutions
class Solution {
// This solution is too difficult to think of, ignore it.
// This solution is similar with 142. Linked List Cycle II (Medium), see also
// https://leetcode.com/problems/linked-list-cycle-ii/
public int findDuplicate(int[] nums) {
int fast = nums[0];
int slow = nums[0];
do {
fast = nums[nums[fast]];
slow = nums[slow];
} while (fast != slow);
slow = nums[0];
while (fast != slow) {
fast = nums[fast];
slow = nums[slow];
}
return fast;
}
}