Stream of Characters in C++

Suppose we want to implement the StreamChecker class as follows −

• StreamChecker(words) − This is the constructor, this initializes the data structure with the given words.

• query(letter) − This returns true when for some k >= 1, the last k characters queried (in order from oldest to newest, including this letter just queried) spell one of the words in the given list.

So, if the input is like word list = ["ce", "g", "lm"], then call query many times for [a,b,c,e,f,g,h,i,j,k,l,m], then the output will be true for e, g, m, and false for others.

To solve this, we will follow these steps −

• Define a node structure, there is an array of 26 nodes, and isEnd flag

• initially isEnd is false, and the child array is filled with null.

• Define a node head

• Make an array of nodes waitList

• Define a function insertNode(), this will take head, s,

  • curr := head

  • for initialize i := 0, when i < size of s, update (increase i by 1), do−

    • x := s[i]

    • if child[x - 'a'] of curr is not-null, then −

      • curr.child[x - 'a'] = create a new node Node

    • curr := curr.child[x - 'a']

  • isEnd of curr := true

• From the initializer do this

  • head := create a new node

  • for initialize i := 0, when i < size of words, update (increase i by 1), do −

    • insertNode(head, words[i])

  • curr := head

• Define a function query(), this will take x,

  • make one array of nodes temp

  • if head.child[x - 'a'], then −

    • insert head at the end of waitList

  • ret := false

  • for initialize i := 0, when i < size of waitList, update (increase i by 1), do −

    • curr := waitList[i]

    • if curr.child[x - 'a'], then

      • curr := child[x - 'a'] of curr

      • insert curr at the end of temp

      • ret := ret OR isEnd of curr

  • swap(temp, waitList)

  • return ret

Let us see the following implementation to get better understanding −

Example

#include <bits/stdc++.h>
using namespace std;
struct Node {
   Node* child[26];
   bool isEnd;
   Node(){
      isEnd = false;
      for (int i = 0; i < 26; i++)
      child[i] = NULL;
   }
};
class StreamChecker {
   public:
   Node* head;
   vector<Node*> waitList;
   void insertNode(Node* head, string& s){
      Node* curr = head;
      for (int i = 0; i < s.size(); i++) {
         char x = s[i];
         if (!curr->child[x - 'a']) {
            curr->child[x - 'a'] = new Node();
         }
         curr = curr->child[x - 'a'];
      }
      curr->isEnd = true;
   }
   StreamChecker(vector<string>& words){
      head = new Node();
      for (int i = 0; i < words.size(); i++) {
         insertNode(head, words[i]);
      }
      Node* curr = head;
   }
   bool query(char x){
      vector<Node*> temp;
      if (head->child[x - 'a']) {
         waitList.push_back(head);
      }
      bool ret = false;
      for (int i = 0; i < waitList.size(); i++) {
         Node* curr = waitList[i];
         if (curr->child[x - 'a']) {
            curr = curr->child[x - 'a'];
            temp.push_back(curr);
            ret |= curr->isEnd;
         }
      }
      swap(temp, waitList);
      return ret;
   }
};
main(){
   vector<string> v = {"ce","g","lm"};
   StreamChecker ob(v);
   cout << (ob.query('a')) << endl;
   cout << (ob.query('b')) << endl;
   cout << (ob.query('c')) << endl;
   cout << (ob.query('e')) << endl;
   cout << (ob.query('f')) << endl;
   cout << (ob.query('g')) << endl;
   cout << (ob.query('h')) << endl;
   cout << (ob.query('i')) << endl;
   cout << (ob.query('j')) << endl;
   cout << (ob.query('k')) << endl;
   cout << (ob.query('l')) << endl;
   cout << (ob.query('m'));
}

Input

"ce","g","lm", query(),

Output

0 0 0 1 0 1 0 0 0 0 0 1