LeetCode #350 - Intersection of Two Arrays II

题目描述：

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?

求两个数组的相同元素，如果相同元素出现多次，那么在结果中也要出现多次。思路和Intersection of Two Arrays一样，用哈希表统计出现次数即可。

class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        unordered_map<int,int> hash;
        for(int i=0;i<nums1.size();i++) hash[nums1[i]]++;
        vector<int> result;
        for(int i=0;i<nums2.size();i++)
            if(hash.count(nums2[i])>0&&hash[nums2[i]]>0) 
            {
                hash[nums2[i]]--;
                result.push_back(nums2[i]);
            }
        return result;
    }
};