src/main/java/com/fishercoder/solutions/_349.java

package com.fishercoder.solutions;

import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;
import java.util.stream.Collectors;

/**
 * 349. Intersection of Two Arrays
 *
 * Given two arrays, write a function to compute their intersection.
 *
 * Example 1:
 * Input: nums1 = [1,2,2,1], nums2 = [2,2]
 * Output: [2]
 *
 * Example 2:
 * Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
 * Output: [9,4]
 *
 * Note:
 * Each element in the result must be unique.
 * The result can be in any order.
 */
public class _349 {

  public static class Solution1 {
    //credit: https://leetcode.com/articles/intersection-of-two-arrays/
    //Time: O(m+n) on average, O(m*n) in worse case
    //Space: O(m+n)
    public int[] intersection(int[] nums1, int[] nums2) {
      Set<Integer> set1 = Arrays.stream(nums1).boxed().collect(Collectors.toSet());
      Set<Integer> set2 = Arrays.stream(nums2).boxed().collect(Collectors.toSet());
      set1.retainAll(set2);
      int[] intersection = new int[set1.size()];
      int i = 0;
      for (int num : set1) {
        intersection[i++] = num;
      }
      return intersection;
    }
  }

  public static class Solution2 {
      //Time: O(nlgn)
    public int[] intersection(int[] nums1, int[] nums2) {
      Arrays.sort(nums2);
      Set<Integer> intersect = new HashSet();
      for (int i : nums1) {
        if (binarySearch(i, nums2)) {
          intersect.add(i);
        }
      }
      int[] result = new int[intersect.size()];
      int i = 0;
      for (int num : intersect) {
        result[i++] = num;
      }
      return result;
    }

    private boolean binarySearch(int i, int[] nums) {
      int left = 0;
      int right = nums.length - 1;
      while (left <= right) {
        int mid = left + (right - left) / 2;
        if (nums[mid] == i) {
          return true;
        } else if (nums[mid] > i) {
          right = mid - 1;
        } else {
          left = mid + 1;
        }
      }
      return false;
    }
  }

  public static class Solution3 {
    //use two pointers
    //credit: https://leetcode.com/problems/intersection-of-two-arrays/discuss/81969/Three-Java-Solutions
    public int[] intersection(int[] nums1, int[] nums2) {
      Arrays.sort(nums1);
      Arrays.sort(nums2);
      int i = 0;
      int j = 0;
      Set<Integer> set = new HashSet<>();
      while (i < nums1.length && j < nums2.length) {
        if (nums1[i] == nums2[j]) {
          set.add(nums1[i++]);
          j++;
        } else if (nums1[i] < nums2[j]) {
          i++;
        } else {
          j++;
        }
      }
      int[] intersection = new int[set.size()];
      int k = 0;
      for (int num : set) {
        intersection[k++] = num;
      }
      return intersection;
    }
  }
}