Given a binary string, count number of substrings that start and end with 1.
Given a binary string, count number of substrings that start and end with 1. For example, if the input string is “00100101”, then there are three substrings “1001”, “100101” and “101”.
Source: Amazon Interview Experience | Set 162
Difficulty Level: Rookie
A Simple Solution is to run two loops. Outer loops picks every 1 as starting point and inner loop searches for ending 1 and increments count whenever it finds 1.
C++
// A simple C++ program to count number of // substrings starting and ending with 1 #include<iostream> using namespace std; int countSubStr(char str[]) { int res = 0; // Initialize result // Pick a starting point for (int i=0; str[i] !='\0'; i++) { if (str[i] == '1') { // Search for all possible ending point for (int j=i+1; str[j] !='\0'; j++) if (str[j] == '1') res++; } } return res; } // Driver program to test above function int main() { char str[] = "00100101"; cout << countSubStr(str); return 0; } |
Java
// A simple C++ program to count number of //substrings starting and ending with 1 class CountSubString { int countSubStr(char str[],int n) { int res = 0; // Initialize result // Pick a starting point for (int i = 0; i<n; i++) { if (str[i] == '1') { // Search for all possible ending point for (int j = i + 1; j< n; j++) { if (str[j] == '1') res++; } } } return res; } // Driver program to test the above function public static void main(String[] args) { CountSubString count = new CountSubString(); String string = "00100101"; char str[] = string.toCharArray(); int n = str.length; System.out.println(count.countSubStr(str,n)); } } |
Python3
# A simple Python 3 program to count number of # substrings starting and ending with 1 def countSubStr(st, n) : # Initialize result res = 0 # Pick a starting point for i in range(0, n) : if (st[i] == '1') : # Search for all possible ending point for j in range(i+1, n) : if (st[j] == '1') : res = res + 1 return res # Driver program to test above function st = "00100101"; list(st) n= len(st) print(countSubStr(st, n), end="") # This code is contributed # by Nikita Tiwari. |
C#
// A simple C# program to count number of // substrings starting and ending with 1 using System; class GFG { public virtual int countSubStr(char[] str, int n) { int res = 0; // Initialize result // Pick a starting point for (int i = 0; i < n; i++) { if (str[i] == '1') { // Search for all possible // ending point for (int j = i + 1; j < n; j++) { if (str[j] == '1') { res++; } } } } return res; } // Driver Code public static void Main(string[] args) { GFG count = new GFG(); string s = "00100101"; char[] str = s.ToCharArray(); int n = str.Length; Console.WriteLine(count.countSubStr(str,n)); } } // This code is contributed by Shrikant13 |
PHP
<?php // A simple PHP program to count number of // substrings starting and ending with 1 function countSubStr($str) { $res = 0; // Initialize result // Pick a starting point for ($i = 0; $i < strlen($str); $i++) { if ($str[$i] == '1') { // Search for all possible // ending point for ($j = $i + 1; $j < strlen($str); $j++) if ($str[$j] == '1') $res++; } } return $res; } // Driver Code $str = "00100101"; echo countSubStr($str); // This code is contributed by ita_c ?> |
Output:
3
Time Complexity of the above solution is O(n2). We can find count in O(n) using a single traversal of input string. Following are steps.
a) Count the number of 1’s. Let the count of 1’s be m.
b) Return m(m-1)/2
The idea is to count total number of possible pairs of 1’s.
C++
// A O(n) C++ program to count number of // substrings starting and ending with 1 #include<iostream> using namespace std; int countSubStr(char str[]) { int m = 0; // Count of 1's in input string // Traverse input string and count of 1's in it for (int i=0; str[i] !='\0'; i++) { if (str[i] == '1') m++; } // Return count of possible pairs among m 1's return m*(m-1)/2; } // Driver program to test above function int main() { char str[] = "00100101"; cout << countSubStr(str); return 0; } |
Java
// A O(n) C++ program to count number of substrings //starting and ending with 1 class CountSubString { int countSubStr(char str[], int n) { int m = 0; // Count of 1's in input string // Traverse input string and count of 1's in it for (int i = 0; i < n; i++) { if (str[i] == '1') m++; } // Return count of possible pairs among m 1's return m * (m - 1) / 2; } // Driver program to test the above function public static void main(String[] args) { CountSubString count = new CountSubString(); String string = "00100101"; char str[] = string.toCharArray(); int n = str.length; System.out.println(count.countSubStr(str, n)); } } |
Python3
# A Python3 program to count number of # substrings starting and ending with 1 def countSubStr(st, n) : # Count of 1's in input string m = 0 # Traverse input string and # count of 1's in it for i in range(0, n) : if (st[i] == '1') : m = m + 1 # Return count of possible # pairs among m 1's return m * (m - 1) // 2 # Driver program to test above function st = "00100101"; list(st) n= len(st) print(countSubStr(st, n), end="") # This code is contributed # by Nikita Tiwari. |
C#
// A O(n) C# program to count // number of substrings starting // and ending with 1 using System; class GFG { int countSubStr(char []str, int n) { int m = 0; // Count of 1's in // input string // Traverse input string and // count of 1's in it for (int i = 0; i < n; i++) { if (str[i] == '1') m++; } // Return count of possible // pairs among m 1's return m * (m - 1) / 2; } // Driver Code public static void Main(String[] args) { GFG count = new GFG(); String strings = "00100101"; char []str = strings.ToCharArray(); int n = str.Length; Console.Write(count.countSubStr(str, n)); } } // This code is contributed by princiraj |
PHP
<?php // A simple PHP program to count number of // substrings starting and ending with 1 function countSubStr($str) { $m = 0; // Initialize result // Pick a starting point for ($i = 0; $i < strlen($str); $i++) { if ($str[$i] == '1') { $m++; } } // Return count of possible // pairs among m 1's return $m * ($m - 1) / 2; } // Driver Code $str = "00100101"; echo countSubStr($str); // This code is contributed // by Akanksha Rai ?> |
Output:
3
This article is contributed by Shivam. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
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