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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
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