` ````
```

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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.

**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.

**Credits:**

Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

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Binary Search
Array
Two Pointers
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