[LeetCode][Python]442. 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.

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：

1.1 Python中有个collections模块，它提供了个类Counter，用来跟踪值出现了多少次。注意key的出现顺序是根据计数的从大到小。它的一个方法most_common(n)返回一个list, list中包含Counter对象中出现最多前n个元素。

1.2 heapq模块有两个函数：nlargest() 和 nsmallest() 可以从一个集合中获取最大或最小的N个元素列表。heapq.nlargest (n, heap) #查询堆中的最大元素，n表示查询元素个数

1.3 使用字典，依次查看数组中的元素，如果在字典中，则放入另外用来统计的list

class Solution(object):
    def findDuplicates(self, nums):
        """
        :type nums: List[int]
        :rtype: List[int]
        """
        return [no for no, count in collections.Counter(nums).items() if count > 1]

Java思路：（我还没想明白:(）
// when find a number i, flip the number at position i-1 to negative.
// if the number at position i-1 is already negative, i is the number that occurs twice.
//The concept here is to negate each number's index as the input is 1 <= a[i] <= n (n = size of array).
// Once a value is negated, if it requires to be negated again then it is a duplicate.

    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> res = new ArrayList<>();
        for(int i=0; i<nums.length; i++){
            int index = Math.abs(nums[i]) - 1;
            if (nums[index] < 0)
                res.add(Math.abs(index + 1));
            nums[index] = -nums[index];
        }
        return res;
    }