LeetCode Two Sum | C++ | KeepLearningEveryday

Two Sum - LeetCode solutions.

This video contains the solution for the problem #TwoSum in two ways. This is one of common questions asked in Telephonic #Interview round of #Microsoft #Amazon #facebook
1. Bruteforce, Time complexity O(n^2)
2. Efficient, Time complexity O(n)

Please do let me know if you have any inputs/concerns for the solution.

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Thank you

[luffy_lyf]

Thank you a lot Nikash! It was such a beginner friendly video.

[mohamedantar]

class Solution {
public:
vector<int> twoSum(vector<int>& nums, int target) {
unordered_map<int, int> ans;
int size = nums.size();
int k ;
vector<int> ret;
for (int i =0; i<size ; i++){
k = target - nums[i];
if (ans.find(k) != ans.end() && ans.find(k)-> second != i){
//then the k does exist in teh map
ret.emplace_back(i);

return ret ;
}
//if you reached here then the k doesn't exist, then add the nums[i]
ans.emplace(nums[i], i );
}
return ret;
}
};

[AdityaSaurabh]

You don't need the second check on line 12 because in a single pass when you are at i, it can never be in the map. It works without the check because you only check against the previous ones and never with yourself.

[michaelhernandez]

wow you made this look easy. I thought we had to iterate through the vector with for(auto iter = nums.begin(); iter != nums.end(); iter++){//logic}
maybe that is when you are messing with a vector holding objects? I need to open up my intro to C++ book again.

[srujanbandam]

i think in condition of maps it should be compared to the element in vector not the index...

[saravanakumar]

Thanks Nikash. Clear Explanation. Keep it up. Looking forward to seeing more videos in Leetcode C++

[mirdulswarup]

please please tell me why are checking the m.end() condition i dont get that, please explain that

[Overrunnerr]

For the first for loop, why do you do “i < size - 1”? I am just confuse about it. For size; would it mean the number of index inside the array of nums? Which is 3. Do you subtract the array as the loops keeps going until the the condition is met where “i” is less than “size - 1”?

[مريممجدي-صح]

What should I learn to be able solving these problems ? (I only learned the basic of c++)

[yogeshyts]

what is the meaning of
m.find(diff)!=m.end()
at 11:44

[rhehebbfb]

#include<iostream>
#include<unordered_map>
#include<vector>

using namespace std;

class Solution {
public:
vector<int> twoSum(vector<int>& nums, int target) {
vector<int>nums_indices;
int size = nums.size();
int second_integer;

unordered_map<int, int>m;

for(int i = 0; i < size; i++){
second_integer = target - nums.at(i);
if(m.find(second_integer) != m.end()){
nums_indices.push_back(i);
-> second);
return nums_indices;
}
}
return nums_indices;
}
};

int main(){
vector<int>nums{2, 7, 11, 15};
int target = 9;
Solution s;
s.twoSum(nums, target);
return 0;
}

[vedikasangle]

Hey Nikash Thank you so much for such easy to understand code but I am getting runtime error when I type the same code. Can you please help me out?

[mrnoobie]

Can u tell me the Y v used two ' return ' in brute -force method.

[adios]

Why do we need to return the vector in if statement (it's not a function either) someone pls help

[prashantpant]

what does m.find(diff)--> second signify? I lost it here.

[tusharshaily]

I didn't get the part of time complexity : Insertion and look up takes log(size) time in map.
and the outer for loop runs N times.
then shouldnt it be Nlog(size) complexity == NlogN.
please do explain

[khushbookumari]

thanks can you recommend books for competitive programming

[madanmohan]

i don't understand how push_back worked there can someone help?

[ananydubey]

Your voice is very low
Next time keep your mic near your mouth !