IntersectionOfTwoArraysII350.java

/**
 * 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?
 *
 */


public class IntersectionOfTwoArraysII350 {
 public int[] intersect(int[] nums1, int[] nums2) {
 if (nums1.length == 0 || nums2.length == 0) return new int[0];
 Map<Integer, Integer> m1 = new HashMap<>();
 for (int i=0; i<nums1.length; i++) {
 m1.put(nums1[i], m1.getOrDefault(nums1[i], 0)+1);
 }

 Map<Integer, Integer> m2 = new HashMap<>();
 for (int i=0; i<nums2.length; i++) {
 m2.put(nums2[i], m2.getOrDefault(nums2[i], 0)+1);
 }

 List<Integer> res = new ArrayList<>();
 if (m1.size() > m2.size()) {
 Map<Integer, Integer> temp = m1;
 m1 = m2;
 m2 = temp;
 }

 for (Map.Entry<Integer, Integer> e: m1.entrySet()) {
 if (m2.containsKey(e.getKey())) {
 int minVal = Math.min(e.getValue(), m2.get(e.getKey()));
 while (minVal > 0) {
 res.add(e.getKey());
 minVal--;
 }
 }
 }

 int[] arr = new int[res.size()];
 for (int i=0; i<res.size(); i++) arr[i] = res.get(i);

 return arr;
 }


 // solution to 3rd follow-up
 public int[] intersect2(int[] nums1, int[] nums2) {
 int len = Math.min(nums1.length, nums2.length);
 int[] arr = new int[len];
 if (len == 0) return arr;

 Map<Integer, Integer> m1 = new HashMap<>();
 for (int i=0; i<nums1.length; i++) {
 m1.put(nums1[i], m1.getOrDefault(nums1[i], 0)+1);
 }

 int size = 0;
 for (int i=0; i<nums2.length; i++) {
 if (m1.containsKey(nums2[i]) && m1.get(nums2[i]) > 0) {
 m1.put(nums2[i], m1.get(nums2[i])-1);
 arr[size] = nums2[i];
 size++;
 }
 }

 return Arrays.copyOfRange(arr, 0, size);
 }


 public int[] intersect3(int[] nums1, int[] nums2) {
 int len = Math.min(nums1.length, nums2.length);
 int[] arr = new int[len];
 if (len == 0) return arr;

 Arrays.sort(nums1);
 Arrays.sort(nums2);

 int i = 0;
 int j = 0;
 int size = 0;
 while (i < nums1.length && j < nums2.length) {
 int n1 = nums1[i];
 int n2 = nums2[j];
 if (n1 == n2) {
 arr[size] = n1;
 i++;
 j++;
 size++;
 } else if (n1 < n2) {
 do {
 i++;
 } while (i < nums1.length && nums1[i] == n1);
 } else {
 do {
 j++;
 } while (j < nums2.length && nums2[j] == n2);
 }

 }

 return Arrays.copyOfRange(arr, 0, size);
 }

}