Program to find start indices of all anagrams of a string S in T in Python

Suppose we have two strings S and T, we have to find all the start indices of S's anagrams in T. The strings consist of lowercase letters only and the length of both strings S and T will not be larger than 20 and 100.

So, if the input is like S = "cab" T = "bcabxabc", then the output will be [0, 1, 5, ], as the substrings "bca", "cab" and "abc".

To solve this, we will follow these steps:

• Define a map m, n := size of s, set left := 0, right := 0, counter := size of p

• define an array ans

• store the frequency of characters in p into the map m

• for right := 0 to n – 1

  • if m has s[right] and m[s[right]] is non zero, then decrease m[s[right]] by 1, decrease counter by 1 and if counter = 0, then insert left into ans

  • otherwise

    • while left < right,

      • if s[left] is not present in m, then increase counter by 1, and increase m[s[left]] by 1

      • increase left by 1

      • if m has s[right] and m[s[right]] is non zero, then decrease right by 1, and come out from the loop

    • if m has no s[left], then set left := right + 1

• return ans

Let us see the following implementation to get better understanding:

Example

#include <bits/stdc++.h>
using namespace std;
void print_vector(vector<auto> v){
   cout << "[";
   for(int i = 0; i<v.size(); i++){
      cout << v[i] << ", ";
   }
   cout << "]"<<endl;
}
class Solution {
public:
   vector<int> findAnagrams(string s, string p) {
      map <char, int> m;
      int n = s.size();
      int left = 0, right = 0;
      int counter = p.size();
      vector <int> ans;
      for(int i = 0; i < p.size(); i++) m[p[i]]++;
      for(int right = 0; right < n; right++){
         if(m.find(s[right]) != m.end() && m[s[right]]){
            m[s[right]]--;
            counter--;
            if(counter == 0)ans.push_back(left);
         } else {
            while(left<right){
               if(m.find(s[left]) != m.end()) {
                  counter++;
                  m[s[left]]++;
               }
               left++;
               if(m.find(s[right]) != m.end() && m[s[right]]){
                  right--;
                  break;
               }
            }
            if(m.find(s[left])==m.end())left = right + 1;
         }
      }
      return ans;
   }
};
main(){
   Solution ob;
   print_vector(ob.findAnagrams("bcabxabc", "cab")) ;
}

Input

"bcabxabc", "cab"

Output

[0, 1, 5, ]