0287. Find the Duplicate Number

https://leetcode.com/problems/find-the-duplicate-number

Description

Given an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive.

There is only one repeated number in nums, return this repeated number.

You must solve the problem without modifying the array nums and uses only constant extra space.

Example 1:

**Input:** nums = [1,3,4,2,2]
**Output:** 2

Example 2:

**Input:** nums = [3,1,3,4,2]
**Output:** 3

Example 3:

**Input:** nums = [1,1]
**Output:** 1

Example 4:

**Input:** nums = [1,1,2]
**Output:** 1

Constraints:

• 1 <= n <= 105

• nums.length == n + 1

• 1 <= nums[i] <= n

• All the integers in nums appear only once except for precisely one integer which appears two or more times.

Follow up:

• How can we prove that at least one duplicate number must exist in nums?

• Can you solve the problem in linear runtime complexity?

ac

class Solution {
    public int findDuplicate(int[] nums) {
        // fast/slow pointers
        int fast = nums[0], slow = nums[0];
        do {
            slow = nums[slow];
            fast = nums[nums[fast]];
        }while (fast != slow);

        int i = nums[0];
        while (i != slow) {
            i = nums[i];
            slow = nums[slow];
        } 

        return i;
    }
}