# Leetcode 442 - Find All Duplicates in an Array

Note:

• To finish with O(1) extra space? We’d better use the orignal array.
• It’s a method called in-place hash?
• Because all number are between [1,n], and the length of nums is n. So the correct order should be [1,2,3..n] where nums[i] = i - 1.
• If nums[i] is not in the correct place, which means nums[i] !== nums[nums[i] - 1]. We should swap them.
• After swapping, don’t move i, we need to do this again on the new nums[i].
• What about nums[i] == nums[nums[i] - 1]:
• If i !== nums[i] - 1, it means we’ve met a num that appears twice.
• If i === nums[i] - 1, it means the num is in the correct place, let’s i++.

Question:

Given an integer array nums of length n where all the integers of nums are in the range [1, n] and each integer appears once or twice, return an array of all the integers that appears twice.

You must write an algorithm that runs in O(n) time and uses only constant extra space.

Example:

Code: