Question

Given two arrays, write a function to compute their intersection.

Example 1:

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

Example 2:

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?

Thinking:

  • Method 1: 双指针
class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        Arrays.sort(nums1);
        Arrays.sort(nums2);
        int index1 = 0, index2 = 0;
        List<Integer> res = new ArrayList<>();
        while(index1 < nums1.length && index2 < nums2.length){
            int a = nums1[index1++], b = nums2[index2++];
            if(a == b)
                res.add(a);
            else if(a < b)
                index2--;
            else
                index1--;
        }
        int[] result = new int[res.size()];
        int i = 0;
        for(int num : res)
            result[i++] = num;
        return result;
    }
}