442. Find All Duplicates in an Array

题意：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.

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

Example:

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

Output:
[2,3]

思路：这题思路和448.Find All Numbers Disappeared in an Array 基本一样，对原数组的索引操作，加过两次的就是重复的数，这个方法需要扫描两遍数组：

class Solution(object):    def findDuplicates(self, nums):        """        :type nums: List[int]        :rtype: List[int]        """        ans = []        for i in xrange(len(nums)):            nums[(nums[i]-1)%len(nums)] += len(nums)        for i in xrange(len(nums)):            if nums[i] > len(nums)<<1 :                ans.append(i+1)        return ans

这个方法更简单一些，不过思路还是一样的，只是对索引对应的数取负来作标志位：

class Solution(object):    def findDuplicates(self, nums):        """        :type nums: List[int]        :rtype: List[int]        """        res = []        for x in nums:            if nums[abs(x)-1] < 0:                res.append(abs(x))            else:                nums[abs(x)-1] *= -1        return res