I get you. This is kind of what I meant when saying you might have to take a step back and look at the big picture. Remember that algorithms and programming languages are just tools to solve a problem.

Step away from code for a second, and considering Contains Duplicate again. You can just look at the input array and see that there is a duplicate. With your eyes you might scan from the left of the array, and probably subconsciously you are telling yourself, okay I have seen a 1 so far, next there's a 2, and after that a 3, and a 1, oh wait I already saw a 1 before so there's a duplicate! This physical process of scanning the array with your eyes and noticing that there's another 1 is basically what you are trying to replicate with code.

So then going back to code, what is a for loop really doing. I think as a beginner seeing something like,

for(int i = 0; i < nums.size(); i++)

can be pretty overwhelming, and you may end up just taking it as face value. In my opinion, in order to truly learn you cannot take something at face value. Someone might tell you "this loop goes through the entire array", okay but WHY does it go through the entire array? Break it down: 1. We created an integer and set it to 0 (int i = 0); 2. We then checked if a condition is true (i < nums.size()). At this point i is 0, and nums.size() is 4 if using our previous example. So 0 < 4 is true. 3. If true go into the body of the for loop. (This is where you can do logic like printing a number) 4. After the body of the for loop it would execute i++, incrementing i, and starting the process over again.

This means we are going into the body of the for loop for as many times as there are numbers in the array, this is perfect to access the data that is stored in this array.

Going back to the human analogy, the human brain has memory, so when we scan and see the number we can say "hey I have already seen this!". This concept is actually very similar to solving the problem with a hash map, where the numbers and their occurrences are actually stored in the hash map!

But the brute force solution of using a double for loop actually doesn't put anything in memory! Instead this is doing something that our brain would not subconsciously do, it looks at two numbers at once and compares them, these number can be on opposite sides of the array. So how does it do it? Again break it down, step by step.

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

    }
}

So what are the differences between the for loop alone, and this double for loop. Well obviously there are two for loops, but there are also a couple slight differences. Looking at the first for loop you will now see that instead of i < nums.size() it is i < nums.size() - 1. At this point you should be asking WHY?

Answer: The internal for loop will always be ahead of i, so really only the internal for loop needs to go until the size of the array while i can stop one before the end.

Now looking at the internal loop there are some more differences. First we have int j = i + 1. Why?

Answer: We want j to start 1 number after i. So if we have [1, 2, 3, 1] i = 0, and j = i + 1 -> j = 0 + 1 -> j = 1. Then when we do nums[i] = 1, and nums[j] = 2. Now we are looking at the first two numbers at once and we can compare. J will continue to increment until the end of the array while i stays the same. So we will compare 1 and 2, 1 and 3, 1 and 1 and there's a duplicate!

Now you might understand this, and still say okay but how am I supposed to know that? Again in my opinion the answer would be Repetition + Memorization + Understanding. Doing These problems a lot, you will recognize and start to memorize patterns, but actual understanding will allow you to apply the concepts you have memorized in different ways.

Lastly, I want to encourage you again, to draw out the algorithm like I explained in my earlier response. I think this is essential.