442. 找出数组中重复的元素 Find All Duplicates in an ArrayGiven an array of integers

Posted 2020-09-03 Long Long Journey

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]

题意：找出数组中重复的元素

关键：1 ≤ a[i] ≤ n  和 appear twice

解法：nums[Math.Abs(nums[i]) - 1] *= -1 如果 nums[Math.Abs(nums[i]) - 1] 小于0 则nums[i]重复

public class Solution {
    public IList<int> FindDuplicates(int[] nums) {
		List<int> list = new List<int>();
		for (int i = 0; i < nums.Length; i++) {
			nums[Math.Abs(nums[i]) - 1] *= -1;
			if (nums[Math.Abs(nums[i]) - 1] > 0) {
				list.Add(Math.Abs(nums[i]));
			}
		}
		return list;
    }
}

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