I am new to Vector while solving Twosum problem from leetcode, encountered following Compilation error :

Line 4: Char 26: error: no viable conversion from 'std::vector<int, std::allocator<int>>::iterator' (aka '__normal_iterator<int *, std::vector<int, std::allocator<int>>>') to 'unsigned int'
    for(unsigned int i=nums.begin();i<nums.end();i++)

Here is my code:

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        for(unsigned int i=nums.begin();i<nums.end();i++)
        {
            for(int j=i+1;j<=nums.end();j++)
            {
                if(nums[i]+nums[j]==target)
                {
                    vector<int> vec;
                    vec[0]=i;
                    vec[1]=j;
                    break;
                }
            }
        }
        return vec;
     }
};

I tried googling this but didn’t get much. Just wanted to know what does this error mean and how to fix it.

>Solution :

Clearly what you meant to write is

    for(unsigned int i=0;i<nums.size();i++)
    {
        for(int j=i+1;j<nums.size();j++)
        {

You are getting confused between iterators and indexes. You also made a mistake using j<= instead of j<.

Also you should note that the type of a vector index is size_t, so the above would be better written as

    for(size_t i=0;i<nums.size();i++)
    {
        for(size_t j=i+1;j<nums.size();j++)
        {

Another serious mistake is here

                vector<int> vec;
                vec[0]=i;
                vec[1]=j;

This is the common but completely unjustified belief that vectors resize themselves when you assign to an index that does not exist. If you want a vector of size 2 then create it so

                vector<int> vec(2);
                vec[0]=i;
                vec[1]=j;

Or create it at size zero (the default) and add the new items using push_back.

                vector<int> vec;
                vec.push_back(i);
                vec.push_back(j);

A final mistake (nothing to do with vectors specifically) is that you’ve declared your vector inside the loop, but you are trying to return it outside the loop. That’s not allowed.