Question

I have the below Problem Statement

PS: Given a string “str” and a Non-Empty substring “sub” ,compute “Recursively” if at least “N” copies of “sub” appear in the “string somewhere”, possibly with “Overlapping”.
N will be non-negative.

Example are as shown below
strCopies("catcowcat", "cat", 2) → true
strCopies("catcowcat", "cow", 2) → false
strCopies("catcowcat", "cow", 1) → true
strCopies("iiijjj", "ii", 2) → true

I have written the code as shown below(without recursion) and is working fine for few test cases,except for others which are marked as FAIL.

:::Code is as shown below:::

public boolean strCopies(String str, String sub, int n) {    
    int len = sub.length();    
    int result=0;    
    if(len>0){    
       int start = str.indexOf(sub);    
       while(start !=-1){    
              result++;    
              start = str.indexOf(sub,start+len);                     
       }
    }          
   if(result==n){
        return true;
   }else return false; 
}

Runs for above code as shown below(Marked in BOLD are FAILED TEST CASES)

Expected This Run
strCopies("catcowcat", "cat", 2) → true true OK
strCopies("catcowcat", "cow", 2) → false false OK
strCopies("catcowcat", "cow", 1) → true true OK
strCopies("iiijjj", "ii", 2) → true false FAIL
strCopies("iiiiij", "iii", 3) → true false FAIL
strCopies("ijiiiiij", "iiii", 2) → true false FAIL

Could you check and let me know what is wrong with the code for FAIL TEST CASES ?Im unable to consider the Overlapping scenarios.

Answer 1

Well, your first problem is that your method isn’t recursive. I suspect you want to work with substring as well as indexOf…

As for why your current method isn’t working, I suspect it’s because you’re using start + len instead of start + 1 to find the next starting position. So when trying to find “ii” in “iii”, you should first look at position 0, then position 1 – currently you’re looking at position 2, which would mean it wouldn’t find the second “ii” starting at 1.