Intersection-of-Two-Arrays-II

LeetCode

第78天。

又是hash,为什么每次都随机到hash.

今天的题目是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?

和昨天的解法有点想,先遍历一个数组，用hash table进行计数，然后再遍历第二个数组，然后实时的更新hash table就好了：

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vector<int> intersec1t(vector<int>& nums1, vector<int>& nums2) {
    unordered_map<int,int> m;
    for(auto i:nums1) m[i]++;
    vector<int> ret;
    for(auto i:nums2) {
        if (m[i] != 0) {
            ret.push_back(i);
            m[i]--;
        }
    }
    return ret;
}

还有就是可以用sort来做：

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vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
    sort(nums1.begin(),nums1.end());
    sort(nums2.begin(),nums2.end());
    int i1,i2;
    vector<int> ret;
    for(i1=0,i2=0;i1 < nums1.size() && i2 < nums2.size();) {
        if (nums1[i1] == nums2[i2]) {
            ret.push_back(nums1[i1]);
            i1++; i2++;
        } else if (nums1[i1] > nums2[i2]) i2++;
        else i1++;
    }
    return ret;
}