README.md

287. 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:

1. You must not modify the array (assume the array is read only).
2. You must use only constant, O(1) extra space.
3. Your runtime complexity should be less than O(n²).
4. There is only one duplicate number in the array, but it could be repeated more than once.

Solutions (Rust)

1. Two Pointers

impl Solution {
    pub fn find_duplicate(nums: Vec<i32>) -> i32 {
        let mut p1 = 0;
        let mut p2 = 0;

        loop {
            p1 = nums[p1 as usize];
            p2 = nums[nums[p2 as usize] as usize];
            if p1 == p2 {
                break;
            }
        }

        p1 = 0;
        while p1 != p2 {
            p1 = nums[p1 as usize];
            p2 = nums[p2 as usize];
        }

        p1
    }
}