Storing new value to array

[William369]

Hi

I am new to Python and I saw this two sum algorithm solution

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        
        prevMap = {}
        
        for i, n in enumerate(nums):
            diff = target - n
            if diff in prevMap:
                return [prevMap[diff], i]
            prevMap[i] = n

In this line prevMap[i] = n I should add a value at index but it return empty array [] or should I call it map or list ? but if I use it like this prevMap[n] = i it work correctly I don understant why ? I am comming from other languages where I store new value into array like this myarray[index] = value; so python is do it in the opposit way ?

[deanhystad]

The algorithm is gibberish. None of it makes sense at all.

What it is trying to do is this:
if a + b = c, then c - a = b.

Keep a record of c - a. If b appears in this record, a + b = c. In your example a dictionary (prevMap) is used to keep the record of c - a. The dictionary should have c - a as the key, and a as the value. The algorithm checks if b matches any of the dictionary keys (if b in prevMap). If it does we know that prevMap[b] + b = c (return prevMap[b], b).

[William369]

The algorithm is gibberish. None of it makes sense at all.

What it is trying to do is this:
if a + b = c, then c - a = b.

Keep a record of c - a. If b appears in this record, a + b = c. In your example a dictionary (prevMap) is used to keep the record of c - a. The dictionary should have c - a as the key, and a as the value. The algorithm checks if b matches any of the dictionary keys (if b in prevMap). If it does we know that prevMap[b] + b = c (return prevMap[b], b).

Can you use the the code variables above to understand ? I only dont understand last line

[William369]

I think he used mapping value:index to return the index for the two numbers

[deanhystad]

You are probably right about that indexing thing. The dictionary is still completely wrong. Should be "prevMap[diff] = i" and "if n in prevMap"". n + diff = target