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Leetcode 350 Intersection Of Two Arrays II

发表于 更新于 分类于 Valine：
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Given two arrays, write a function to compute their intersection.

Example 1:

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Input: nums1 = [1,2,2,1], nums2 = [2,2]
Output: [2,2]

Example 2:

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Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
Output: [4,9]

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?

哈希表

先用哈希表储存某个数组的信息, 然后遍历另一个数组, 得到交集, 也能用排序做, 但是时间复杂度更高.空间复杂度一样, 都是O(m + n)

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class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        Map<Integer, Integer> map1 = new HashMap<>();
        for(int i : nums1)
        {
            map1.putIfAbsent(i,0);
            map1.put(i, map1.get(i) + 1);
        }
        List<Integer> ans = new ArrayList<>();
        for(int i : nums2)
        {
            if(map1.get(i) != null && map1.get(i) != 0)
            {
                map1.put(i, map1.get(i) - 1);
                ans.add(i);
            }
        }
        int[] ret = new int[ans.size()];
        int k = 0;
        for(int i : ans)
            ret[k++] = i;
        return ret;
    }
}