INTERVIEW QUESTION C(ZOHO)

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QUESTION

Find if a String2 is substring of String1. If it is, return the index of the first occurrence. else return -1.

Eg 1:Input:
        String 1: test123string
         String 2: 123
        Output: 4
Eg 2: Input:
        String 1: testing12
        String 2: 1234 
        Output: -1

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PROGRAM:

#include <stdio.h>
#include<string.h>
int main()
{
 char a[100],b[100];
 int i,j,flag=0,n,k;
 scanf("%s %s",a,b);
 for(i=0;i<strlen(a);i++)
 {
     k=i;
     for(j=0;j<strlen(b);j++)
     {
         if(a[k]!=b[j])
         {
         break;
         }
         if(a[k]==b[j] && j==(strlen(b)-1))
         {
         flag=1;
         n=i+1;
         break;
         }
         k++;
     }
     if(flag==1)
     {
     printf("%d",n);
     break;
     }
 }
 if(flag==0)
 {
     printf("-1");
 }
}