三种方法求解两个数组的交集

package com.Test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

//求两个数组的交集
public class FindIntersectionBetweenTwoArrays {
	
	//算法一：暴力搜索，时间复杂度O(n^2)，空间复杂度O(1)
	public ArrayList getIntersection(int[] arr, int[] arr2) {
		
		ArrayList res = new ArrayList();
		if(arr == null || arr.length == 0 || arr2 == null || arr2.length == 0) {
			return res;
		}
		
		for(int i = 0;i < arr.length;i ++) {
			int temp = arr[i];
			for(int j = 0;j < arr.length;j ++) {
				if(arr2[j] == temp && !res.contains(temp)) {
					res.add(temp);
					break;
				}
			}
		}
		return res;
	}
	
	//算法二：先排序，然后定义两个指针，时间复杂度O(nlogn) (排序)，空间复杂度O(1)
	public ArrayList getIntersection2(int[] arr, int[] arr2) {
		
		ArrayList res = new ArrayList();
		if(arr == null || arr.length == 0 || arr2 == null || arr2.length == 0) {
			return res;
		}
		
		Arrays.sort(arr);
		Arrays.sort(arr2);
		int i = 0;
		int j = 0;
		while(i < arr.length && j < arr2.length) {
			if(arr[i] < arr2[j]) {
				i ++;
			}
			else if(arr[i] > arr2[j]) {
				j ++;
			}
			else {
				if(!res.contains(arr[i])) {
					res.add(arr[i]);
				}
				i ++;
				j ++;
			}
		}
		return res;
	}
	
	//算法三：map计数，时间复杂度O(n)，空间复杂度O(n)，空间换时间
	public ArrayList getIntersection3(int[] arr, int[] arr2) {
		
		ArrayList res = new ArrayList();
		if(arr == null || arr.length == 0 || arr2 == null || arr2.length == 0) {
			return res;
		}
		
		Map map = new HashMap();
		for(int i = 0;i < arr.length;i ++) {
			int key = arr[i];
			if(map.containsKey(key)) {
				int value = map.get(key);
				value ++;
				map.put(key, value);
			}
			else {
				map.put(key, 1);
			}
		}
		for(int i = 0;i < arr2.length;i ++) {
			int key = arr2[i];
			if(map.containsKey(key) && !res.contains(key)) {
				res.add(key);
			}
		}
		return res;
		
	}
	
	public static void main(String[] args) {
		
		int[] arr = {1,2,5,3,3,4};
		int[] arr2 = {2,3,3,4,5,6};
		FindIntersectionBetweenTwoArrays fb = new FindIntersectionBetweenTwoArrays();
		List res = fb.getIntersection3(arr, arr2);
		System.out.println(res);
	}
}