LeetCode 350. Intersection of Two Arrays II（数组交集）

原题网址：https://leetcode.com/problems/intersection-of-two-arrays-ii/

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

Example:
Given nums1 = [1, 2, 2, 1], nums2 = [2, 2], return [2, 2].

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 num2'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?

方法一：哈希映射计数。

public class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        Map map1 = new HashMap<>();
        for(int num: nums1) {
            Integer count = map1.get(num);
            if (count == null) count = 1; else count ++;
            map1.put(num, count);
        }
        List list2 = new ArrayList<>();
        for(int num: nums2) {
            Integer count = map1.get(num);
            if (count == null) continue;
            list2.add(num);
            count --;
            if (count == 0) map1.remove(num); else map1.put(num, count);
        }
        int[] result = new int[list2.size()];
        for(int i=0; i

方法二：先排序再查找。

public class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        Arrays.sort(nums1);
        Arrays.sort(nums2);
        List list = new ArrayList<>();
        int i=0, j=0;
        while (i