Intersection of Two Arrays II

Desicription

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

Example 1:

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Input: nums1 = [1,2,2,1], nums2 = [2,2]
Output: [2,2]

Example 2:

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Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
Output: [4,9]

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?

Solution

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class Solution {
public:
std::vector<int> intersect(std::vector<int>& nums1, std::vector<int>& nums2) {
auto count1 = std::unordered_map<int, int>();
auto count2 = std::unordered_map<int, int>();

for(auto num : nums1) {
count1[num]++;
}
for(auto num : nums2) {
count2[num]++;
}

auto res = std::vector<int>();
for(auto num : count1) {
if(count2.find(num.first) != count2.end()) {
for(int i = 0; i < std::min(num.second, count2[num.first]); i++) {
res.push_back(num.first);
}
}
}

return res;
}
};