Find All Duplicates in an Array

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements that appear twice in this array.

Example

Example1

Input:
[4,3,2,7,8,2,3,1]
Output:
[2,3]

Example2

Input:
[10,2,5,10,9,1,1,4,3,7]
Output:
[1,10]

Challenge

Could you do it without extra space and in O(n) runtime?

分析

把当前数abs(x)-1作为Index, nums[index] flop. 如果当前数字已经是负数，说明index+1已经出现过

class Solution:
    """
    @param nums: a list of integers
    @return: return a list of integers
    """
    def findDuplicates(self, nums):
        # write your code here
        res = []
        for x in nums:
            idx = abs(x)-1
            if nums[idx]< 0:
                res.append(idx + 1)
            else:
                nums[idx] *= -1
        return res