get difference between two arrays (including duplicates)

Question

I see a lot of posts about how to get the difference and symmetric difference of an array in javascript, but I haven\'t found anything on how to find the difference, includi

Answer 1

You can do it the following steps (O(n)).

Let a and b are two arrays

Step 1. create map hash_map of array a value as key and number occurrences of this key as value.

Step 2. push all the elements of array b in result which are not in a using hash_map.

Step 3. push all the elements of array a in result which are not in b using hash_map.

Here is complete code

function diff(a, b) {
    //Step 1 starts here
	var hash_map = a.reduce(function(map, key) {
		map[key] = map[key] ? (map[key]+1) : 1;
		return map;
	}, {});
    //Step 1 ends here
    //Step 2 starts here
	var result = b.filter(function(val) {
		if(hash_map[val]) {
			hash_map[val] = hash_map[val]-1;
			return false;
		}
		return true;
	});
    //Step 2 ends hers
    //Step 3 starts here
	Object.keys(hash_map).forEach(function(key) {
		while (hash_map[key]) {
			result.push(key);
			hash_map[key] = hash_map[key]-1;
		}
	});
    //Step 3 ends here
	return result;
}

console.log(diff([1],[1,1,2]));
console.log(diff([1,1,1],[1,1,1,1,1,2]));
console.log(diff([1,1,3,4],[1,2,3]));
console.log(diff([1,1,1,1,1,2], [1, 2, 1, 1, 3]));