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intersection-of-two-arrays-ii

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Intersection of Two Arrays II – LeetCode 350

Problem

Description

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

Example

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

Answer

Original

Code

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

思路

优先排序然后进行遍历检查，时间复杂度$O(nlog{n})$，空间复杂度$O(1)$。
耗时$7$ ms，排名$72.80\%$

Better

思路

使用hashmap存储出现的次数，用空间换时间，时间复杂度$O(n)$，空间复杂度$O(n)$。
耗时$7$ ms，排名$72.80\%$

Code

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class Solution {
public:
    vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {
        unordered_map<int, int> m;
        vector<int> res;
        for (auto a : nums1) 
            ++m[a];
        for (auto a : nums2)
            if (m[a]-- > 0) res.push_back(a);
        return res;
    }
};