Array Single Duplicate LeetCode (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(n^2).
• There is only one duplicate number in the array, but it could be repeated more than once.

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Solution: