Leetcode Intersection of Two Arrays i,ii查找两个数组的公共元素

/***************************************************************************************** Given two arrays, write a function to compute their intersection.** Example:* Given nums1 = [1, 2, 2, 1], nums2 = [2, 2], return [2].** Note:* Each element in the result must be unique.* The result can be in any order.****************************************************************************************///用set#include "stdafx.h"#include <iostream>#include <vector>#include <set>using namespace std;vector<int> intersection(vector<int> &num1, vector<int> &num2){    set<int>s1, s2;    vector<int>result;    auto iter = num1.begin();    for (; iter != num1.end(); ++iter)        s1.insert(num1[*iter]);    auto iter2 = num2.begin();    for (; iter2 != num2.end(); ++iter2)        if (s1.find(*iter2) != s1.end())//说明在s1中找到了相同的数            s2.insert(*iter2);//插入s2中    auto iter3 = s2.begin();//s2中全是查找相同的元素，且没有重复的值    for (; iter3 != s2.end(); ++iter3)        result.push_back(*iter3);    return result;}

用哈希表

class Solution {    public:    vector<int> intersection(vector<int>& nums1, vector<int>& nums2) {       unordered_map<int,int> m;       vector<int> res;       for (int i=0;i<nums1.size();i++)           m[nums1[i]]=1;       for (int i=0;i<nums2.size();i++) {           if (m[nums2[i]]>0) {               res.push_back(nums2[i]);               m[nums2[i]]=0;           }       }       return res;    }};

主要下面题目的不同

/*****************************************************************************
*
* 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 num2’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?
*
*****************************************************************************/

哈希表,时间复杂度O(N),空间O(N)

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

排序，无额外空间复杂度

class Solution {public:   vector<int> intersect(vector<int>& nums1, vector<int>& nums2) {      vector<int> res;      sort(nums1.begin(),nums1.end());      sort(nums2.begin(),nums2.end());      int n1=nums1.size(),n2=nums2.size();      int i=0,j=0;      while (i<n1&&j<n2) {          if (nums1[i]==nums2[j]) {              res.push_back(nums1[i]);              i++;j++;          }          else if (nums1[i]>nums2[j])            j++;          else             i++;      }    return res;}};