[LeetCode] 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 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?

使用map记录nums1中元素出现的次数，然后遍历nums2，判断元素是否在map中出现，如果出现则加入结果数组中，使该元素在m中的值-1。

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        vector<int> res;
        unordered_map<int, int> m;
        for (int num1 : nums1)
            m[num1]++;
        for (int num2 : nums2) 
            if (m[num2]) {
                res.push_back(num2);
                m[num2]--;
            }
        return res;
    }
};
// 6 ms

相关题目：Intersection of Two Arrays