[leetcode] 350. 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.

Solution

Idea: hash table O(m + n) Space: O(m + n)

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

Follow up 1: What if the given array is already sorted? How would you optimize your algorithm? 

Solution: O(max(m, n) log(max(m, n))) Space: O(m + n)

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