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Count of pairs (A, B) in range 1 to N such that last digit of A is equal to the first digit of B

  • Last Updated : 19 May, 2021

Given a number N, the task is to find the number of pairs (A, B) in the range [1, N] such that the last digit of A is equal to the first digit of B, and the first digit of A is equal to the last digit of B.
Examples: 
 

Input: N = 25 
Output: 17 
Explanation: 
The pairs are: 
(1, 1), (1, 11), (2, 2), (2, 22), (3, 3), (4, 4), (5, 5), (6, 6), (7, 7), (8, 8), (9, 9), (11, 1), (11, 11), (12, 21), (21, 12), (22, 2), (22, 22)
Input: N = 100 
Output: 108 
 

 

Approach: For each pair of integers (i, j)(0 ≤ i, j ≤ 9), let us define ci, j (1 ≤ k ≤ N) which is the count of the first digit of k is equal to i, and the last digit is equal to j. By using ci, j, the answer for the problem can be calculated by i=09j=09 ci, j * cj, i .
Below is the implementation of the above approach: 
 

CPP




// C++ program to implement the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to Count of pairs (A, B) in range 1 to N
int pairs(int n)
{
    vector<vector<int> > c(10, vector<int>(10, 0));
 
    int tmp = 1;
 
    // count C i, j
    for (int i = 1; i <= n; i++) {
        if (i >= tmp * 10)
            tmp *= 10;
        c[i / tmp][i % 10]++;
    }
 
    // Calculate number of pairs
    long long ans = 0;
    for (int i = 1; i < 10; i++)
        for (int j = 1; j < 10; j++)
            ans += (long long)c[i][j] * c[j][i];
 
    return ans;
}
 
// Driver code
int main()
{
    int n = 25;
 
    // Function call
    cout << pairs(n);
 
    return 0;
}

Java




// Java program to implement the above approach
 
class GFG{
  
// Function to Count of pairs (A, B) in range 1 to N
static int pairs(int n)
{
    int [][]c = new int[10][10];
  
    int tmp = 1;
  
    // count C i, j
    for (int i = 1; i <= n; i++) {
        if (i >= tmp * 10)
            tmp *= 10;
        c[i / tmp][i % 10]++;
    }
  
    // Calculate number of pairs
    int ans = 0;
    for (int i = 1; i < 10; i++)
        for (int j = 1; j < 10; j++)
            ans += c[i][j] * c[j][i];
  
    return ans;
}
  
// Driver code
public static void main(String[] args)
{
    int n = 25;
  
    // Function call
    System.out.print(pairs(n));
  
}
}
 
// This code is contributed by Rajput-Ji

Python3




# Python3 program to implement the above approach
 
# Function to Count of pairs (A, B) in range 1 to N
def pairs(n):
    c = [[0 for i in range(10)] for i in range(10)]
 
    tmp = 1
 
    # count C i, j
    for i in range(1, n + 1):
        if (i >= tmp * 10):
            tmp *= 10
        c[i // tmp][i % 10] += 1
 
    # Calculate number of pairs
    ans = 0
    for i in range(1, 10):
        for j in range(1, 10):
            ans += c[i][j] * c[j][i]
 
    return ans
 
# Driver code
if __name__ == '__main__':
    n = 25
 
    # Function call
    print(pairs(n))
 
# This code is contributed by mohit kumar 29   

C#




// C# program to implement the above approach
using System;
 
class GFG{
   
// Function to Count of pairs (A, B) in range 1 to N
static int pairs(int n)
{
    int [,]c = new int[10, 10];
   
    int tmp = 1;
   
    // count C i, j
    for (int i = 1; i <= n; i++) {
        if (i >= tmp * 10)
            tmp *= 10;
        c[i / tmp, i % 10]++;
    }
   
    // Calculate number of pairs
    int ans = 0;
    for (int i = 1; i < 10; i++)
        for (int j = 1; j < 10; j++)
            ans += c[i, j] * c[j, i];
   
    return ans;
}
   
// Driver code
public static void Main(String[] args)
{
    int n = 25;
   
    // Function call
    Console.Write(pairs(n));
   
}
}
 
// This code is contributed by Rajput-Ji

Javascript




<script>
 
// JavaScript program for the above approach
 
// Function to Count of pairs (A, B) in range 1 to N
function pairs(n)
{
    let c = new Array(10);
    for (var i = 0; i < c.length; i++) {
    c[i] = new Array(2);
    }
    for (var i = 0; i < c.length; i++) {
    for (var j = 0; j < c.length; j++) {
    c[i][j] = 0;
    }
    }
    
    let tmp = 1;
    
    // count C i, j
    for (let i = 1; i <= n; i++) {
        if (i >= tmp * 10)
            tmp *= 10;
        c[Math.floor(i / tmp)][i % 10]++;
    }
    
    // Calculate number of pairs
    let ans = 0;
    for (let i = 1; i < 10; i++)
        for (let j = 1; j < 10; j++)
            ans += c[i][j] * c[j][i];
    
    return ans;
}
 
// Driver code
 
         let n = 25;
    
    // Function call
    document.write(pairs(n));
 
</script>
Output: 



17

 

Time Complexity: O(N)
 

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