Topic
- Hash Table
- Two Pointers
- Binary Search
- Sort
Description
https://leetcode.com/problems/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.
Analysis
方法一：我写的，使用Java8的特性
方法二：set的retainAll方法
方法三：传统做法
方法四：使用Java8的特性，比方法一更精简
方法五：位图法
还有其他方法，如，将实参的两数组分别排序，然后用双指针，或二分查找寻找出共同元素。
Submission
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;
import java.util.stream.Collectors;
public class IntersectionOfTwoArrays {
// 方法一：我写的，使用 java8的特性
public int[] intersection1(int[] nums1, int[] nums2) {
Set<Integer> set1 = Arrays.stream(nums1).boxed().collect(Collectors.toSet());
Set<Integer> tmp = new HashSet<>();
for (int x : nums2)
if (set1.contains(x))
tmp.add(x);
return tmp.stream().mapToInt(Integer::intValue).toArray();
}
// 方法二：set的retainAll方法
public int[] intersection2(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);
return set1.stream().mapToInt(Integer::intValue).toArray();
}
// 方法三：传统做法
public int[] intersection3(int[] nums1, int[] nums2) {
Set<Integer> set = new HashSet<>();
Set<Integer> intersect = new HashSet<>();
for (int i = 0; i < nums1.length; i++) {
set.add(nums1[i]);
}
for (int i = 0; i < nums2.length; i++) {
if (set.contains(nums2[i])) {
intersect.add(nums2[i]);
}
}
int[] result = new int[intersect.size()];
int i = 0;
for (Integer num : intersect) {
result[i++] = num;
}
return result;
}
// 方法四：使用Java8的精简版
public int[] intersection4(int[] nums1, int[] nums2) {
Set<Integer> set = Arrays.stream(nums2).boxed().collect(Collectors.toSet());
return Arrays.stream(nums1).distinct().filter(e -> set.contains(e)).toArray();
}
// 方法五：位图法
public int[] intersection5(int[] nums1, int[] nums2) {
int max = Integer.MIN_VALUE, min = Integer.MAX_VALUE;
int[] ans = new int[nums1.length];
int idx = 0;
for (int i : nums1) {
if (i > max)
max = i;
if (i < min)
min = i;
}
for (int i : nums2) {
if (i > max)
max = i;
if (i < min)
min = i;
}
boolean[] map = new boolean[max - min + 1];
for (int i : nums1)
map[i - min] = true;
for (int i : nums2) {
if (map[i - min]) {
ans[idx++] = i;
map[i - min] = false;
}
}
return Arrays.copyOf(ans, idx);
}
}
Test
import static org.junit.Assert.*;
import java.util.Arrays;
import org.junit.Test;
public class IntersectionOfTwoArraysTest {
@Test
public void test1() {
IntersectionOfTwoArrays obj = new IntersectionOfTwoArrays();
assertArrayEquals(new int[] {
2 }, //
obj.intersection1(new int[] {
1, 2, 2, 1 }, new int[] {
2, 2 }));
int[] expected = {
9, 4 };
int[] actual = obj.intersection1(new int[] {
4, 9, 5 }, new int[] {
9, 4, 9, 8, 4 });
Arrays.sort(expected);
Arrays.sort(actual);
assertArrayEquals(expected, actual);
}
@Test
public void test2() {
IntersectionOfTwoArrays obj = new IntersectionOfTwoArrays();
assertArrayEquals(new int[] {
2 }, //
obj.intersection2(new int[] {
1, 2, 2, 1 }, new int[] {
2, 2 }));
int[] expected = {
9, 4 };
int[] actual = obj.intersection2(new int[] {
4, 9, 5 }, new int[] {
9, 4, 9, 8, 4 });
Arrays.sort(expected);
Arrays.sort(actual);
assertArrayEquals(expected, actual);
}
@Test
public void test3() {
IntersectionOfTwoArrays obj = new IntersectionOfTwoArrays();
assertArrayEquals(new int[] {
2 }, //
obj.intersection3(new int[] {
1, 2, 2, 1 }, new int[] {
2, 2 }));
int[] expected = {
9, 4 };
int[] actual = obj.intersection3(new int[] {
4, 9, 5 }, new int[] {
9, 4, 9, 8, 4 });
Arrays.sort(expected);
Arrays.sort(actual);
assertArrayEquals(expected, actual);
}
@Test
public void test4() {
IntersectionOfTwoArrays obj = new IntersectionOfTwoArrays();
assertArrayEquals(new int[] {
2 }, //
obj.intersection4(new int[] {
1, 2, 2, 1 }, new int[] {
2, 2 }));
int[] expected = {
9, 4 };
int[] actual = obj.intersection4(new int[] {
4, 9, 5 }, new int[] {
9, 4, 9, 8, 4 });
Arrays.sort(expected);
Arrays.sort(actual);
assertArrayEquals(expected, actual);
}
@Test
public void test5() {
IntersectionOfTwoArrays obj = new IntersectionOfTwoArrays();
assertArrayEquals(new int[] {
2 }, //
obj.intersection5(new int[] {
1, 2, 2, 1 }, new int[] {
2, 2 }));
int[] expected = {
9, 4 };
int[] actual = obj.intersection5(new int[] {
4, 9, 5 }, new int[] {
9, 4, 9, 8, 4 });
Arrays.sort(expected);
Arrays.sort(actual);
assertArrayEquals(expected, actual);
}
}