[Solved] How to implement this Python code in Swift?

First you should do is to properly define type of the graph, because unlike Python you have to specify types in Swift in declaration:

var graph: [String: [String: Int]] // Dictionary(hash table) with keys of type String and values of type Dictionary<String, Int>

Then you should initialize graph with some initial value, because in Swift you always explicitly initialize non-nullable variables:

graph = [:] // empty dictionary, in Python it's {}

Declaration and initialization can be in one line, so you may just do this:

var graph: [String: [String: Int]] = [:]

Then your code snippet, with little changes:

graph["start"] = [:]
graph["start"]?["a"] = 6 // ? can be replaced with ! here, because we know for sure "start" exists
graph["start"]?["b"] = 2 // but for simple tutorial purposes, I chose to use ? here

But it would be better if you would define "start" value at once:

graph["start"] = [
    "a": 6,
    "b": 2
]

Or even do it for whole graph:

let graph: [String: [String: Int]] = [
    "start": [
        "a": 6,
        "b": 2
    ]
]