Pankaj Tanwar
Published on

Concatenation of Array.

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Problem of the day - Concatenation of Array

Tag - Easy

Problem says, Given an integer array nums of length n, you want to create an array ans of length 2n where ans[i] == nums[i] and ans[i + n] == nums[i] for 0 <= i < n (0-indexed).

Specifically, ans is the concatenation of two nums arrays.

Yay, pretty easy!

Create a new array of length 2*n and, iterate over the original array and push element to i and i+n index. And tada, it's done.

I directly coded the solution.

class Solution {
public:
    vector<int> getConcatenation(vector<int>& nums) {
        int size = nums.size();
        vector<int> res(2*size, 0);
        for(int i=0;i<nums.size();i++) {
            res[i] = nums[i];
            res[i+size] = nums[i];
        }
        return res;
    }
};

Time and space both are o(n). And total run time on leetcode is 8ms, with 12.4 MB space.

Umm, can this solution be improved further? Can I avoid using an extra array? What if I push to the same array?

class Solution {
public:
    vector<int> getConcatenation(vector<int>& nums) {
        int n = nums.size();
        for(int i=0;i<n;i++) {
            nums.push_back(nums[i]);
        }
        return nums;
    }
};

Yay! I just removed extra array. Total run time is now 4ms on leetcode.

Well, I decided to try STL here. Just to make this code fancy.

class Solution {
public:
    vector<int> getConcatenation(vector<int>& nums) {
        nums.insert(nums.end(), nums.begin(), nums.end());
        return nums;
    }
};

A pretty neat and clear solution. For those who are not aware of insert() function in C++, it is used for inserting values in a vector. First argument is the location and second is the values.

You can read more about it here

By the way, this method is slow. It took 12ms and 12.7MB memory.

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