350. Intersection of Two Arrays II(easy)

Easy

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Given two arrays, write a function to compute their intersection.

Example 1:

Input: nums1 = [1,2,2,1], nums2 = [2,2]

Output: [2,2]

Example 2:

Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]

Output: [4,9]

Note:

• Each element in the result should appear as many times as it shows in both arrays.
• The result can be in any order.

Follow up:

• What if the given array is already sorted? How would you optimize your algorithm?
• What if nums1's size is small compared to nums2's size? Which algorithm is better?
• What if elements of nums2 are stored on disk, and the memory is limited such that you cannot load all elements into the memory at once?

先排序法：

C++：

/*
 * @Autor: SourDumplings
 * @Date: 2019-09-14 10:13:30
 * @Link: https://github.com/SourDumplings/
 * @Email: changzheng300@foxmail.com
 * @Description: https://leetcode.com/problems/intersection-of-two-arrays-ii/
 */

class Solution
{
public:
    vector<int> intersect(vector<int> &nums1, vector<int> &nums2)
    {
        sort(nums1.begin(), nums1.end());
        sort(nums2.begin(), nums2.end());
        vector<int> res;
        int j = 0;
        int n1 = nums1.size(), n2 = nums2.size();
        for (int i = 0; i < n1; i++)
        {
            while (j < n2 && nums2[j] < nums1[i])
                ++j;
            if (j == n2)
            {
                break;
            }
            else if (nums2[j] == nums1[i])
            {
                res.push_back(nums2[j++]);
            }
        }
        return res;
    }
};

 哈希表法：

Java：

import java.util.Hashtable;
import java.util.Map;
import java.util.Vector;

/*
 * @Autor: SourDumplings
 * @Date: 2019-09-14 10:23:05
 * @Link: https://github.com/SourDumplings/
 * @Email: changzheng300@foxmail.com
 * @Description: https://leetcode.com/problems/intersection-of-two-arrays-ii/
 */

class Solution
{
    public int[] intersect(int[] nums1, int[] nums2)
    {
        Map<Integer, Integer> map1 = new Hashtable<>();
        Map<Integer, Integer> map2 = new Hashtable<>();
        for (int i : nums1)
        {
            if (map1.containsKey(i))
            {
                map1.replace(i, map1.get(i) + 1);
            }
            else
            {
                map1.put(i, 1);
            }
        }
        for (int i : nums2)
        {
            if (map2.containsKey(i))
            {
                map2.replace(i, map2.get(i) + 1);
            }
            else
            {
                map2.put(i, 1);
            }
        }
        Vector<Integer> temp = new Vector<>();
        for (int i : map1.keySet())
        {
            if (map2.containsKey(i))
            {
                int n1 = map1.get(i);
                int n2 = map2.get(i);
                int n = n1 < n2 ? n1 : n2;
                while (n-- > 0)
                {
                    temp.add(i);
                }
            }
        }
        int n = temp.size();
        int[] res = new int[n];
        int i = 0;
        for (int num : temp)
        {
            res[i++] = num;
        }
        return res;
    }
}