【LeetCode】442. Find All Duplicates in an Array【M】【60】

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]

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主要就是不用额外的空间 以及O(n)。

所以就用数组内部数值进行标记，每次访问，把数值对应的index对应的数标记为负数，要是第二次遇见负数，那就说明出现两次了

class Solution(object):
    def findDuplicates(self, nums):
        res = []
        for i in xrange(len(nums)):

            idx = abs(nums[i]) - 1
            if nums[idx] < 0:
                res += idx+1,
            nums[idx] = -abs(nums[idx])
            #print idx,nums,res
        return res