Wrap every continuous instance of given word with some given string

Given two string S1 and S2. The task is to wrap every instance of string S2 in string S1 with some string on either side.
Note: Here we will use underscore(_) for the wrapping part. It can be anything as per need, for example some HTML tag, white space, etc.
Examples:

Input: S1 = “examwill be a examexam”, S2 = “exam” 
Output: “_exam_will be a _examexam_” 
Explanation:
String S2 is “exam“, so wrap occurrence of S2 with underscore. Since, last two occurrence overlaps(in string examexam), so merge both the occurrence and wrap by putting underscore to farther left and farther right of merged substring.
Input: str = “abcabcabcabc”, substr = “abc” 
Output: “_abcabcabcabc_” 

Approach: The idea is to simply get the location of all instances of string S2 in S1 and add underscore at either end of the occurrence and if two or more instances of S2 overlaps then, add underscore at either end of the merged substring. Below are the steps: 

1. Get the location of all instances of the string S2 in the main string S1. For that traverse string S1 one character at a time and call substring-matching function, find().
2. Create a 2D array of locations(say arr[][]), where each subarray holds the starting and ending indices of a specific instance of the string S2 in the string S1.
3. Merge the overlapping starting and ending index intervals stored in arr[][].
4. After the above steps all the overlapping indexes are merged. Now traverse the given string and intervals at the same time and create a new string by wrapping underscores.
5. Print the new string after the above step.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Function Definitions
vector<vector<int> > getLocations(string str,
                                  string subStr);

vector<vector<int> > collapse(
    vector<vector<int> > locations);

string underscorify(string str,
                    vector<vector<int> > locations);

// Function that creates the string by
// wrapping the underscore
string underscorifySubstring(string str,
                             string subStr)
{

    // Function Call to intervals of
    // starting and ending index
    vector<vector<int> > locations
        = collapse(getLocations(str, subStr));

    // Function Call
    return underscorify(str, locations);
}

// Function finds all starting and ending
// index of the substring in given string
vector<vector<int> > getLocations(string str,
                                  string subStr)
{
    vector<vector<int> > locations{};
    int startIdx = 0;

    int L = subStr.length();

    // Traverse string str
    while (startIdx < str.length()) {

        // Find substr
        int nextIdx = str.find(subStr,
                               startIdx);

        // If location found, then insert
        // pair int location[]
        if (nextIdx != string::npos) {

            locations.push_back({ nextIdx,
                                  (nextIdx + L) });

            // Update the start index
            startIdx = nextIdx + 1;
        }
        else {
            break;
        }
    }
    return locations;
}

// Function to merge the locations
// of substrings that overlap each
// other or sit next to each other
vector<vector<int> > collapse(
    vector<vector<int> > locations)
{
    if (locations.empty()) {
        return locations;
    }

    // 2D vector to store the merged
    // location of substrings
    vector<vector<int> > newLocations{ locations[0] };

    vector<int>* previous = &newLocations[0];

    for (int i = 1; i < locations.size(); i++) {

        vector<int>* current = &locations[i];

        // Condition to check if the
        // substring overlaps
        if (current->at(0) <= previous->at(1)) {
            previous->at(1) = current->at(1);
        }
        else {
            newLocations.push_back(*current);
            previous
                = &newLocations[newLocations.size() - 1];
        }
    }
    return newLocations;
}

// Function creates a new string with
// underscores added at correct positions
string underscorify(string str,
                    vector<vector<int> > locations)
{
    int locationsIdx = 0;
    int stringIdx = 0;
    bool inBetweenUnderscores = false;
    vector<string> finalChars{};
    int i = 0;

    // Traverse the string and check
    // in locations[] to append _
    while (stringIdx < str.length()
           && locationsIdx < locations.size()) {

        if (stringIdx
            == locations[locationsIdx][i]) {

            // Insert underscore
            finalChars.push_back("_");
            inBetweenUnderscores
                = !inBetweenUnderscores;

            // Increment location index
            if (!inBetweenUnderscores) {
                locationsIdx++;
            }
            i = i == 1 ? 0 : 1;
        }

        // Create string s
        string s(1, str[stringIdx]);

        // Push the created string
        finalChars.push_back(s);
        stringIdx++;
    }

    if (locationsIdx < locations.size()) {
        finalChars.push_back("_");
    }
    else if (stringIdx < str.length()) {
        finalChars.push_back(str.substr(stringIdx));
    }

    // Return the resultant string
    return accumulate(finalChars.begin(),
                      finalChars.end(), string());
}

// Driver Code
int main()
{
    // Given string S1 and S2
    string S1 = "examwill be a examexam";
    string S2 = "exam";

    // Function Call
    cout << underscorifySubstring(S1, S2);
    return 0;
}

Java

// Java program for the above approach
import java.io.*;
import java.util.*;

class GFG
{

    // Function that creates the string by
    // wrapping the underscore
    static String underscorifySubstring(String str,
                                        String subStr)
    {

        // Function Call to intervals of
        // starting and ending index
        List<List<Integer> > locations
            = collapse(getLocations(str, subStr));

        // Function Call
        return underscorify(str, locations);
    }

    // Function finds all starting and ending
    // index of the substring in given string
    static List<List<Integer> > getLocations(String str,
                                             String subStr)
    {
        @SuppressWarnings("unchecked")
        List<List<Integer> > locations = new ArrayList();
        int startIdx = 0;

        int L = subStr.length();

        // Traverse string str
        while (startIdx < str.length())
        {

            // Find substr
            int nextIdx = str.indexOf(subStr, startIdx);

            // If location found, then insert
            // pair int location[]
            if (nextIdx != -1)
            {

                locations.add(Arrays.asList(new Integer[] {
                    nextIdx, nextIdx + L }));

                // Update the start index
                startIdx = nextIdx + 1;
            }
            else
            {
                break;
            }
        }
        return locations;
    }

    // Function to merge the locations
    // of substrings that overlap each
    // other or sit next to each other
    static List<List<Integer> >
    collapse(List<List<Integer> > locations)
    {
        if (locations.size() == 0)
        {
            return locations;
        }

        // 2D vector to store the merged
        // location of substrings
        @SuppressWarnings("unchecked")
        List<List<Integer> > newLocations = new ArrayList();
        newLocations.add(locations.get(0));

        List<Integer> previous = locations.get(0);

        for (int i = 1; i < locations.size(); i++)
        {

            List<Integer> current = locations.get(i);

            // Condition to check if the
            // substring overlaps
            if (current.get(0) <= previous.get(1))
            {
                previous.set(1, current.get(1));
            }
            else
            {
                newLocations.add(current);
                previous = newLocations.get(
                    newLocations.size() - 1);
            }
        }
        return newLocations;
    }

    // Function creates a new string with
    // underscores added at correct positions
    static String
    underscorify(String str, List<List<Integer> > locations)
    {
        int locationsIdx = 0;
        int stringIdx = 0;
        boolean inBetweenUnderscores = false;
        StringBuilder finalChars = new StringBuilder();
        int i = 0;

        // Traverse the string and check
        // in locations[] to append _
        while (stringIdx < str.length()
               && locationsIdx < locations.size()) {

            if (stringIdx
                == locations.get(locationsIdx).get(i)) {

                // Insert underscore
                finalChars.append("_");
                inBetweenUnderscores
                    = !inBetweenUnderscores;

                // Increment location index
                if (!inBetweenUnderscores)
                {
                    locationsIdx++;
                }
                i = i == 1 ? 0 : 1;
            }

            // Push the string
            finalChars.append(str.charAt(stringIdx));
            stringIdx++;
        }

        if (locationsIdx < locations.size()) {
            finalChars.append("_");
        }
        else if (stringIdx < str.length()) {
            finalChars.append(str.substring(stringIdx));
        }

        // Return the resultant string
        return finalChars.toString();
    }

    public static void main(String[] args)
    {
        // Given string S1 and S2
        String S1 = "examwill be a examexam";
        String S2 = "exam";

        // Function Call
        System.out.print(underscorifySubstring(S1, S2));
    }
}

// This code is contributed by jithin

Python3

# Python  program for the above approach

# Function that creates the string by
# wrapping the underscore
def underscorify_substring(string, sub_str):

    # Function Call to intervals of starting and ending index
    locations = collapse(get_locations(string, sub_str))

    # Function Call
    return underscorify(string, locations)

# Function finds all starting and ending index of the substring in given string
def get_locations(string, sub_str):
    locations = []
    start_idx = 0
    l = len(sub_str)

    # Traverse string
    while start_idx < len(string):
        # Find sub_str
        next_idx = string.find(sub_str, start_idx)

        # If location found, then insert pair int location[]
        if next_idx != -1:
            locations.append([next_idx, next_idx + l])

            # Update the start index
            start_idx = next_idx + 1
        else:
            break
    return locations

# Function to merge the locations of substrings that overlap each other or sit next to each other


def collapse(locations):
    if not locations:
        return locations

    # 2D list to store the merged location of substrings
    new_locations = [locations[0]]
    previous = locations[0]

    for i in range(1, len(locations)):
        current = locations[i]

        # Condition to check if the substring overlaps
        if current[0] <= previous[1]:
            previous[1] = current[1]
        else:
            new_locations.append(current)
            previous = new_locations[-1]
    return new_locations

# Function creates a new string with underscores added at correct positions
def underscorify(string, locations):
    locations_idx = 0
    string_idx = 0
    in_between_underscores = False
    final_chars = []
    i = 0

    # Traverse the string and check in locations[] to append _
    while string_idx < len(string) and locations_idx < len(locations):
        if string_idx == locations[locations_idx][i]:
            # Insert underscore
            final_chars.append("_")
            in_between_underscores = not in_between_underscores

            # Increment location index
            if not in_between_underscores:
                locations_idx += 1
            i = 1 if i == 0 else 0

        # Push the string
        final_chars.append(string[string_idx])
        string_idx += 1

    if locations_idx < len(locations):
        final_chars.append("_")
    elif string_idx < len(string):
        final_chars.append(string[string_idx:])

    # Return the resultant string
    return "".join(final_chars)


# Given string S1 and S2
s1 = "examwill be a examexam"
s2 = "exam"

# Function Call
print(underscorify_substring(s1, s2))

# This code is contributed by rutikbhosale

Javascript

// JavaScript program for the above approach

// Function that creates the string by
// wrapping the underscore
function underscorify_substring(string, sub_str) {

// Function Call to intervals of starting and ending index
const locations = collapse(get_locations(string, sub_str));

// Function Call
return underscorify(string, locations);
}

// Function finds all starting and ending index of the substring in given string
function get_locations(string, sub_str) {
const locations = [];
let start_idx = 0;
const l = sub_str.length;

// Traverse string
while (start_idx < string.length) {
// Find sub_str
const next_idx = string.indexOf(sub_str, start_idx);

// If location found, then insert pair into locations[]
if (next_idx != -1) {
  locations.push([next_idx, next_idx + l]);

  // Update the start index
  start_idx = next_idx + 1;
} else {
  break;
}
}
return locations;
}

// Function to merge the locations of substrings that overlap each other or sit next to each other
function collapse(locations) {
if (!locations.length) {
return locations;
}

// 2D array to store the merged location of substrings
const new_locations = [locations[0]];
let previous = locations[0];

for (let i = 1; i < locations.length; i++) {
const current = locations[i];

// Condition to check if the substring overlaps
if (current[0] <= previous[1]) {
  previous[1] = current[1];
} else {
  new_locations.push(current);
  previous = new_locations[new_locations.length - 1];
}
}
return new_locations;
}

// Function creates a new string with underscores added at correct positions
function underscorify(string, locations) {
let locations_idx = 0;
let string_idx = 0;
let in_between_underscores = false;
const final_chars = [];
let i = 0;

// Traverse the string and check in locations[] to append _
while (string_idx < string.length && locations_idx < locations.length) {
if (string_idx === locations[locations_idx][i]) {
// Insert underscore
final_chars.push("_");
in_between_underscores = !in_between_underscores;

  // Increment location index
  if (!in_between_underscores) {
    locations_idx++;
  }
  i = i === 0 ? 1 : 0;
}

// Push the string
final_chars.push(string[string_idx]);
string_idx++;
}

if (locations_idx < locations.length) {
final_chars.push("_");
} else if (string_idx < string.length) {
final_chars.push(string.slice(string_idx));
}

// Return the resultant string
return final_chars.join("");
}

// Given string S1 and S2
const s1 = "examwill be a examexam";
const s2 = "exam";

// Function Call
console.log(underscorify_substring(s1, s2));

// This code is contributed by rutikbhosale

C#

using System;
using System.Collections.Generic;

// Define a class named GFG
class GFG {
    // Define a static method to underscore the given
    // substring in the given string
    static string UnderscorifySubstring(string str,
                                        string subStr)
    {
        // Get the locations of all occurrences of the given
        // substring in the given string
        List<List<int> > locations
            = Collapse(GetLocations(str, subStr));
        // Underscore the given substring in the given
        // string based on the locations of its occurrences
        return Underscorify(str, locations);
    } // Define a static method to get the locations of all
      // occurrences of the given substring in the given
      // string
    static List<List<int> > GetLocations(string str,
                                         string subStr)
    {
        // Initialize a list to store the locations of all
        // occurrences of the given substring in the given
        // string
        List<List<int> > locations = new List<List<int> >();
        // Initialize the starting index for searching the
        // next occurrence of the given substring
        int startIdx = 0;
        // Get the length of the given substring
        int L = subStr.Length;

        // Loop until there are no more occurrences of the
        // given substring in the given string
        while (startIdx < str.Length) {
            // Get the index of the next occurrence of the
            // given substring in the given string
            int nextIdx = str.IndexOf(subStr, startIdx);
            // If the next occurrence of the given substring
            // is found
            if (nextIdx != -1) {
                // Add the start and end indices of the next
                // occurrence of the given substring to the
                // list of locations
                locations.Add(
                    new List<int>{ nextIdx, nextIdx + L });
                // Update the starting index for searching
                // the next occurrence of the given
                // substring
                startIdx = nextIdx + 1;
            }
            // If there are no more occurrences of the given
            // substring in the given string
            else {
                // Break out of the loop
                break;
            }
        }

        // Return the list of locations of all occurrences
        // of the given substring in the given string
        return locations;
    }

    // Define a static method to collapse the overlapping
    // locations of all occurrences of the given substring
    // in the given string
    static List<List<int> >
    Collapse(List<List<int> > locations)
    {
        // If there are no locations to collapse
        if (locations.Count == 0) {
            // Return the original list of locations
            return locations;
        }

        // Initialize a new list to store the collapsed
        // locations
        List<List<int> > newLocations
            = new List<List<int> >();
        // Add the first location to the new list
        newLocations.Add(locations[0]);
        // Initialize a variable to store the previous
        // location
        List<int> previous = locations[0];

        // Loop through all the remaining locations
        for (int i = 1; i < locations.Count; i++) {
            // Get the current location
            List<int> current = locations[i];
            // If the current location overlaps with the
            // previous location
            if (current[0] <= previous[1]) {
                // Update the end index of the previous
                // location to include the current location
                previous[1] = current[1];
            }
            // If the current location does not overlap with
            // the previous location
            else {
                // Add the current location to the new list
                newLocations.Add(current);
                // Update the previous location to be the
                // last location in the new list
                previous
                    = newLocations[newLocations.Count - 1];
            }
        }

        // Return the list of collapsed locations
        return newLocations;
    }

    static string Underscorify(string str,
                               List<List<int> > locations)
    {
        int locationsIdx = 0;
        int stringIdx = 0;
        bool inBetweenUnderscores = false;
        var finalChars = new System.Text.StringBuilder();
        int i = 0;

        while (stringIdx < str.Length
               && locationsIdx < locations.Count) {
            if (stringIdx == locations[locationsIdx][i]) {
                finalChars.Append("_");
                inBetweenUnderscores
                    = !inBetweenUnderscores;
                if (!inBetweenUnderscores) {
                    locationsIdx++;
                }
                i = i == 1 ? 0 : 1;
            }
            finalChars.Append(str[stringIdx]);
            stringIdx++;
        }

        if (locationsIdx < locations.Count) {
            finalChars.Append("_");
        }
        else if (stringIdx < str.Length) {
            finalChars.Append(str.Substring(stringIdx));
        }

        return finalChars.ToString();
    }

    static void Main(string[] args)
    {
        string S1 = "examwill be a examexam";
        string S2 = "exam";
        Console.WriteLine(UnderscorifySubstring(S1, S2));
    }
}

Output:

_exam_will be a _examexam_

Time Complexity: O(N * M), where N and M is the length of the string S1 and S2 respectively. 
Auxiliary Space: O(N), where N is the length of string S1