# leetcode: 2 sum - think about DICTIONARIES

Things I learnt:

### **When there is mapping to be done, think about a** `DICTIONARY`

* Here, we have to use the VALUES of the array, e.g. \[1, 2, 3, 4\]
    
    * Figure out which numbers add up to 6 = 2 + 4
        
* But we want to output the INDEX of the array
    
* So we need to map the values, 2 and 4, to their indices
    
* Whenever mapping is needed, think of using a DICTIONARY
    

### When thinking about `for` loops and lots of iterations, think about a `DICTIONARY`

Brute force:

* I knew I’d need to write some for loops and call stuff again and again til I got the solution.
    
* I also knew this would be poorly optimized for big O notation (time and space)
    
* I thought I’d optimised it because I was reducing the size of the array each time, but, even still, it wasn’t great apparently.
    

### Big O notation

* Time is more important than Space (memory / storage)
    
* Often, the brute force will have
    
    * Time = O(n²) and Space = O(1)
        
    * The largest possible time
        
    * The smallest possible space, because you’re not storing anything
        
* Time is MUCH more important to optimise, because memory is cheap and enormous
    
    * Often, there is a way to flip **Time = O(n²) and Space = O(1)** to **Time = O(n) and Space = O(n)**