G面经prepare: set difference

给你A{1,2,3,4,4,5}， B{2,4}，求A-B={1,3,4,5},很简单. visit 1

只用一个HashMap

package TwoSets;
import java.util.*;

public class Solution {
    public ArrayList<Integer> findDiff(int[] arr1, int[] arr2) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        HashMap<Integer, Integer> m1 = new HashMap<Integer, Integer>();
        for (int item1 : arr1) {
            if (!m1.containsKey(item1)) {
                m1.put(item1, 1);
            }
            else {
                m1.put(item1, m1.get(item1)+1);
            }
        }
        for (int item2 : arr2) {
            if (m1.containsKey(item2)) {
                m1.put(item2, m1.get(item2)-1);
            }
            if (m1.get(item2) == 0) m1.remove(item2);
        }
        for (int elem : m1.keySet()) {
            int num = m1.get(elem);
            while (num > 0) {
                res.add(elem);
                num--;
            }
        }
        return res;
    }
    

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Solution sol = new Solution();
        ArrayList<Integer> res = sol.findDiff(new int[]{1,2,3,4,4,5}, new int[]{2,4});
        System.out.println(res);
    }

}