I am solving the leetcode question 249. Group Shifted Strings. Can someone please explain how the keys are stored in the hashMap?
We can shift a string by shifting each of its letters to its successive letter.
For example, "abc" can be shifted to be "bcd". We can keep shifting the string to form a sequence. For example, we can keep shifting "abc" to form the sequence: "abc" -> "bcd" -> ... -> "xyz".
Given an array of strings strings, group all strings[i] that belong to the same shifting sequence. You may return the answer in any order.
Input: strings = ["abc","bcd","acef","xyz","az","ba","a","z"] Output: [["acef"],["a","z"],["abc","bcd","xyz"],["az","ba"]] func groupStrings(_ strings: [String]) -> [[String]] { var dict = [[Int]: [String]]() for word in strings { var key = [0] if word.count > 1 { var a = Array(word) let pivot = Int(a[0].asciiValue!) - 97 for i in 1..<a.count { let index = (Int(a[i].asciiValue!) - 97 + 26 - pivot) % 26 key.append(index) } } if var array = dict[key] { array.append(word) dict[key] = array } else { dict[key] = [word] } } return dict.keys.compactMap { dict[$0] } } "[[0, 2, 4, 5]: ["acef"], [0, 25]: ["az", "ba"], [0]: ["a", "z"], [0, 1, 2]: ["abc", "bcd", "xyz"]]" 1 Answer
Your question is not much clear but I believe u are requesting an explanation to the groupStrings function.
In your question u have to find the shifting sequences. eg : abc -> bcd -> cde ..... -> xyz.
So the logic behind this is checking the ASCII values of each character. Because if two strings form a shifting sequence the differences between the ASCII values of any consecutive pair of their characters are the same.
For an example we know abc bcd xyz are in a shifting sequence`.
Characters ASCCI values Difference from the 1st value a b c 97 98 99. 0 1 2 b c d 98 99 100. 0 1 2 x y z 120 121 122. 0 1 2 Basically what happening in your function is creating a dictionary which contains those differences as keys and strings as values.
func groupStrings(_ strings: [String]) -> [[String]] { var dict = [[Int]: [String]]() // initialize the dictionary for word in strings { var key = [0] if word.count > 1 { var a = Array(word) //create an array from the charachters of the word let pivot = Int(a[0].asciiValue!) - 97 // get the ASCII value of the 1st lestter // we substract 97 because for english letters ASCII values start from 97. // by substracting 97 we can map the ASCCII of a to 0, b to 1 .... z to 26 for i in 1..<a.count { // calulate the differences between the ASCII values of any consecutive pairs let index = (Int(a[i].asciiValue!) - 97 + 26 - pivot) % 26 key.append(index) } } // here check if there is an already a difference sequence in the dictionary //if true we append the word to that exsiting key value pair //else create a new key value pair if var array = dict[key] { array.append(word) dict[key] = array } else { dict[key] = [word] } } return dict.keys.compactMap { dict[$0] } } 
