LeetCode-442. Find All Duplicates in an Array C#

转载于 2017-03-16 23:39:00 发布 · 62 阅读

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原文链接：http://www.cnblogs.com/MiaBlog/p/6562293.html

文章标签：

#runtime

本文介绍了一种在整数数组中查找重复元素的有效算法，该算法能在O(n)时间内完成任务且不使用额外空间。通过将已出现过的元素所在位置的数值标记为负数，若第二次遇到同一元素时其位置数值已经为负，则将其加入结果列表。

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]

Solution: Similar to Leetcode-448

Mark the value of already apeared index to negative, then if the value appeared second time the index value should be negative, add it to the result list.

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