Find the intersection of arrays

1. Intersection of two numbers

349. Intersection of Two Arrays

Difficulty Easy 716

Given two arrays  nums1 and sum  nums2 , return  their intersection  . Each element in the output must be  unique  . We can  ignore the order of the output results  .

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]
 Explanation: [4,9] is also passable

hint:

• 1 <= nums1.length, nums2.length <= 1000
• 0 <= nums1[i], nums2[i] <= 1000

Method 1: Complete through set and filter

/**
 * @param {number[]} nums1
 * @param {number[]} nums2
 * @return {number[]}
 */
var intersection = function(nums1, nums2) {
    return Array.from(new Set(nums1.filter(num1=>nums2.includes(num1))))
};

Method 2: After sorting, it is done by double pointer

/**
 * @param {number[]} nums1
 * @param {number[]} nums2
 * @return {number[]}
 */
var intersection = function(nums1, nums2) {
    nums1.sort((a,b)=>a-b)
    nums2.sort((a,b)=>a-b)
    const common = []
    let num1Index = 0, num2Index = 0
    // 这里是 && 而不是 ||； 只要有一个遍历完就结束
    while(num1Index < nums1.length && num2Index < nums2.length) {
        const num1 = nums1[num1Index]
        const num2 = nums2[num2Index]
        if(num1 === num2) {
            num1Index++
            num2Index++
            !common.includes(num1) && common.push(num1)
        }
        if(num1 > num2) {
            num2Index++
        }
        if(num1 < num2) {
            num1Index++
        }
    }
    return common
};

2. Multi-array intersection

Analysis: Multi-array is to continue to compare the results of the first two merged arrays. It can be realized by reduce.

function getIntersect(...arrs) {
    return arrs.reduce(function(prev,cur){
	return [...new Set(cur.filter((item)=>prev.includes(item)))]
    })
}