350. Intersection of Two Arrays II

题目：

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 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?

链接：

https://leetcode.com/problems/intersection-of-two-arrays-ii/#/description

3/12/2017

public class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        HashMap<Integer, Integer> h1 = new HashMap<Integer, Integer>();
        ArrayList<Integer> a = new ArrayList<Integer>();

        for (int i = 0; i < nums1.length; i++) {
            if (h1.containsKey(nums1[i])) {
                h1.put(nums1[i], h1.get(nums1[i]) + 1);
            } else {
                h1.put(nums1[i], 1);
            }
        }
        for (int i = 0; i < nums2.length; i++) {
            if (h1.containsKey(nums2[i])) {
                if (h1.get(nums2[i]) > 0) {
                    a.add(nums2[i]);
                    h1.put(nums2[i], h1.get(nums2[i]) - 1);
                } else {
                    h1.remove(nums2[i]);
                }
            }
        }
        int ret[] = new int[a.size()];
        int index = 0;
        for (Integer i: a) {
            ret[index++] = i;
        }
        return ret;
    }
}

followup：

1. 如果输入是sort的，可以用2个指针完成

2. 如果nums1的值的个数远小于nums2，还是hashmap方法好吧？

3. 可以取决于nums1的大小，如果nums1小的话可以用它存入hash；否则external sort2个数组，再来比较

看讨论别人用Python写的非常简洁，用的是collections.Counter。虽然平时用collections比较多，不过这个却从来没有用过。另外除非面试官很了解Python，否则还是解释起来比较麻烦。面试时候尝试用Java会比较好些。