Check if a cell can be visited more than once in a String

Given a string which consists of either ‘.’ or any number. A ‘.’ in the string means that the cell is empty and if there is a number x in any cell, it means one can move x steps to right or left within the string.

The task is to check if any cell in the string can be visited more than once. If so, print YES otherwise print NO.

Examples:



Input : str = ".2...2.."
Output: YES
The fourth cell can be visited twice. One way to reach 
the fourth cell is from 2nd cell by moving 2 steps to right
and another way to reach fourth cell is by moving 2 steps 
left from cell 6.

Input : str = ".2...1"
Output: NO
None of the cells in the given string 
can be visited more than once.

The idea is to take an array visited[] to keep track of the number of times i-th cell of the string can be visited. Now traverse the string and check if the current character is a ‘.’ or a number x. If the current character is a ‘.’ then do nothing otherwise if it is a number then increase the count of visits in the visited array within the range [i-x, i+x] by 1.

Finally, traverse the visited[] array and check if any cell is visited more than once.

Below is the implementation of the above approach:

C++

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// C++ program to check if any cell of the
// string can be visited more than once
  
#include <bits/stdc++.h>
using namespace std;
  
// Function to check if any cell can be
// visited more than once
bool checkIfOverlap(string str)
{
    int len = str.length();
  
    // Array to mark cells
    int visited[len + 1] = { 0 };
  
    // Traverse the string
    for (int i = 0; i < len; i++) {
        if (str[i] == '.')
            continue;
  
        // Increase the visit count of the left and right
        // cells within the array which can be visited
        for (int j = max(0, i - str[i]); j <= min(len, i + str[i]); j++)
            visited[j]++;
    }
  
    for (int i = 0; i < len; i++) {
        // If any cell can be visited more than once
        // Return True
        if (visited[i] > 1) {
            return true;
        }
    }
  
    return false;
}
  
// Driver code
int main()
{
    string str = ".2..2.";
  
    if (checkIfOverlap(str))
        cout << "YES";
    else
        cout << "NO";
  
    return 0;
}

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Java

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// Java program to check if any cell of the
// string can be visited more than once
  
import java.io.*;
  
class GFG {
  
// Function to check if any cell can be
// visited more than once
static boolean checkIfOverlap(String str)
{
    int len = str.length();
  
    // Array to mark cells
    int []visited =  new int[len +1];
  
    // Traverse the string
    for (int i = 0; i < len; i++) {
        if (str.charAt(i)== '.')
            continue;
  
        // Increase the visit count of the left and right
        // cells within the array which can be visited
        for (int j = Math.max(0, i - str.charAt(i)); j <= Math.min(len, i + str.charAt(i)); j++)
            visited[j]++;
    }
  
    for (int i = 0; i < len; i++) {
        // If any cell can be visited more than once
        // Return True
        if (visited[i] > 1) {
            return true;
        }
    }
  
    return false;
}
  
// Driver code
  
    public static void main (String[] args) {
        String str = ".2..2.";
  
    if (checkIfOverlap(str))
        System.out.println("YES");
    else
        System.out.print("NO");
    }
}
  
// This code is contributed by inder_verma..

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Python 3

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# Python3 program to check if 
# any cell of the string can 
# be visited more than once
  
# Function to check if any cell 
# can be visited more than once 
def checkIfOverlap(str) :
  
    length = len(str)
  
    # Array to mark cells
    visited = [0] * (length + 1)
  
    # Traverse the string 
    for i in range(length) :
        if str[i] == "." :
            continue
  
        # Increase the visit count 
        # of the left and right cells
        # within the array which can 
        # be visited 
        for j in range(max(0, i - ord(str[i]), 
                       min(length, i + 
                       ord(str[i])) + 1)) :
            visited[j] += 1
  
    # If any cell can be visited 
    # more than once, Return True 
    for i in range(length) :
  
        if visited[i] > 1 :
            return True
  
    return False
  
# Driver code     
if __name__ == "__main__" :
  
    str = ".2..2."
  
    if checkIfOverlap(str) :
        print("YES")
                          
    else :
        print("NO")
                      
# This code is contributed 
# by ANKITRAI1

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C#

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// C# program to check if any 
// cell of the string can be
// visited more than once
using System;
  
class GFG 
{
  
// Function to check if any cell 
// can be visited more than once
static bool checkIfOverlap(String str)
{
    int len = str.Length;
  
    // Array to mark cells
    int[] visited = new int[len + 1];
  
    // Traverse the string
    for (int i = 0; i < len; i++) 
    {
        if (str[i]== '.')
            continue;
  
        // Increase the visit count of
        // the left and right cells
        // within the array which can be visited
        for (int j = Math.Max(0, i - str[i]); 
                 j <= Math.Min(len, i + str[i]); j++)
            visited[j]++;
    }
  
    for (int i = 0; i < len; i++) 
    {
        // If any cell can be visited 
        // more than once, Return True
        if (visited[i] > 1)
        {
            return true;
        }
    }
  
    return false;
}
  
// Driver code
public static void Main ()
{
    String str = ".2..2.";
  
    if (checkIfOverlap(str))
        Console.Write("YES");
    else
        Console.Write("NO");
}
}
  
// This code is contributed 
// by Akanksha Rai(Abby_akku)

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PHP

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<?php 
// PHP program to check if any 
// cell of the string can be 
// visited more than once
  
// Function to check if any cell 
// can be visited more than once
function checkIfOverlap($str)
{
    $len = strlen($str);
  
    // Array to mark cells
    $visited = array_fill(0, $len + 1, NULL);
  
    // Traverse the string
    for ($i = 0; $i < $len; $i++)
    {
        if ($str[$i] == '.')
            continue;
  
        // Increase the visit count of the 
        // left and right cells within the 
        // array which can be visited
        for ($j = max(0, $i - $str[$i]); 
             $j <= min($len, $i + $str[$i]); 
             $j++)
            $visited[$j]++;
    }
  
    for ($i = 0; $i < $len; $i++) 
    {
        // If any cell can be visited 
        // more than once, Return True
        if ($visited[$i] > 1) 
        {
            return true;
        }
    }
  
    return false;
}
  
// Driver code
$str = ".2..2.";
  
if (checkIfOverlap($str))
    echo "YES";
else
    echo "NO";
  
// This code is contributed
// by ChitraNayal
?>

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Output:

YES


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