# Kth Smallest Element of a Matrix of given dimensions filled with product of indices

Given an integer **K** and a matrix of size **N x M**, where each matrix element is equal to the product of its indices(**i * j**), the task is to find the **K ^{th} Smallest element** in the given Matrix.

**Examples:**

Input:N = 2, M = 3, K = 5Output:4Explanation:

The matrix possible for given dimensions is {{1, 2, 3}, {2, 4, 6}}

Sorted order of the matrix elements: {1, 2, 2, 3, 4, 6}

Therefore, the 5^{th}smallest element is4.Input:N = 1, M = 10, K = 8Output:8Explanation:

The matrix possible for given dimensions is {{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}}

Therefore, the 8^{th}smallest element is8.

**Naive Approach: **The simplest approach is to store all the elements of the matrix in an array and then find the **K ^{th}** smallest element by sorting the array.

**Time Complexity:**O(N×M×log(N×M))

**Auxiliary Space:**O(N×M)**Efficient Approach:**

To optimize the naive approach the idea is to use the Binary Search algorithm. Follow the steps below to solve the problem:

- Initialize
as**low****1**andas**high****N×M,**as the**K**smallest element lies between^{th}**1**and**N×M.** - Find the
**mid****low**elements**high***.* - If the number of elements
*less*thanis**mid***greater*than or equal to**K,**then updateto**high**as the**mid-1****Kth**smallest element lies betweenand**low****mid.** - If the number of elements
*less*thanis**mid***less*than**K,**then updateto**low**as the**mid+1****Kth**smallest element lies betweenand**mid****high.** - As the elements in the
row are the multiple of**ith**, the number of elements less than**i**in the**mid**row can be calculated easily by**ith**So, the time complexity to find the number of elements less than**min(mid/i, M).**can be done in only**mid**.**O(N)** - Perform binary search till
is**low***less*than or equal toand return**high**as the**high + 1****Kth**smallest element of the matrix**N×M.**

Below is the implementation of the above approach:

## C++

`// C++ program for the above approach` `#include <bits/stdc++.h>` `using` `namespace` `std;` `#define LL long long` `// Function that returns true if the` `// count of elements is less than mid` `bool` `countLessThanMid(LL mid, LL N,` ` ` `LL M, LL K)` `{` ` ` `// To store count of elements` ` ` `// less than mid` ` ` `LL count = 0;` ` ` `// Loop through each row` ` ` `for` `(` `int` `i = 1;` ` ` `i <= min((LL)N, mid); ++i) {` ` ` `// Count elements less than` ` ` `// mid in the ith row` ` ` `count = count + min(mid / i, M);` ` ` `}` ` ` `if` `(count >= K)` ` ` `return` `false` `;` ` ` `else` ` ` `return` `true` `;` `}` `// Function that returns the Kth` `// smallest element in the NxM` `// Matrix after sorting in an array` `LL findKthElement(LL N, LL M, LL K)` `{` ` ` `// Initialize low and high` ` ` `LL low = 1, high = N * M;` ` ` `// Perform binary search` ` ` `while` `(low <= high) {` ` ` `// Find the mid` ` ` `LL mid = low + (high - low) / 2;` ` ` `// Check if the count of` ` ` `// elements is less than mid` ` ` `if` `(countLessThanMid(mid, N, M, K))` ` ` `low = mid + 1;` ` ` `else` ` ` `high = mid - 1;` ` ` `}` ` ` `// Return Kth smallest element` ` ` `// of the matrix` ` ` `return` `high + 1;` `}` `// Driver Code` `int` `main()` `{` ` ` `LL N = 2, M = 3;` ` ` `LL ` `int` `K = 5;` ` ` `cout << findKthElement(N, M, K) << endl;` ` ` `return` `0;` `}` |

## Java

`// Java program to implement` `// the above approach` `class` `GFG{` ` ` `// Function that returns true if the` `// count of elements is less than mid` `public` `static` `boolean` `countLessThanMid(` `int` `mid, ` `int` `N,` ` ` `int` `M, ` `int` `K)` `{` ` ` ` ` `// To store count of elements` ` ` `// less than mid` ` ` `int` `count = ` `0` `;` ` ` `// Loop through each row` ` ` `for` `(` `int` `i = ` `1` `;` ` ` `i <= Math.min(N, mid); ++i)` ` ` `{` ` ` ` ` `// Count elements less than` ` ` `// mid in the ith row` ` ` `count = count + Math.min(mid / i, M);` ` ` `}` ` ` `if` `(count >= K)` ` ` `return` `false` `;` ` ` `else` ` ` `return` `true` `;` `}` `// Function that returns the Kth` `// smallest element in the NxM` `// Matrix after sorting in an array` `public` `static` `int` `findKthElement(` `int` `N, ` `int` `M, ` `int` `K)` `{` ` ` ` ` `// Initialize low and high` ` ` `int` `low = ` `1` `, high = N * M;` ` ` `// Perform binary search` ` ` `while` `(low <= high)` ` ` `{` ` ` ` ` `// Find the mid` ` ` `int` `mid = low + (high - low) / ` `2` `;` ` ` `// Check if the count of` ` ` `// elements is less than mid` ` ` `if` `(countLessThanMid(mid, N, M, K))` ` ` `low = mid + ` `1` `;` ` ` `else` ` ` `high = mid - ` `1` `;` ` ` `}` ` ` `// Return Kth smallest element` ` ` `// of the matrix` ` ` `return` `high + ` `1` `;` `}` `// Driver code` `public` `static` `void` `main(String[] args)` `{` ` ` `int` `N = ` `2` `, M = ` `3` `;` ` ` `int` `K = ` `5` `;` ` ` `System.out.println(findKthElement(N, M, K));` `}` `}` `// This code is contributed by divyeshrabadiya07` |

## Python3

`# Python3 program for the above approach` `# Function that returns true if the` `# count of elements is less than mid` `def` `countLessThanMid(mid, N, M, K):` ` ` ` ` `# To store count of elements` ` ` `# less than mid` ` ` `count ` `=` `0` ` ` `# Loop through each row` ` ` `for` `i ` `in` `range` `(` `1` `, ` `min` `(N, mid) ` `+` `1` `):` ` ` `# Count elements less than` ` ` `# mid in the ith row` ` ` `count ` `=` `count ` `+` `min` `(mid ` `/` `/` `i, M)` ` ` ` ` `if` `(count >` `=` `K):` ` ` `return` `False` ` ` `else` `:` ` ` `return` `True` `# Function that returns the Kth` `# smallest element in the NxM` `# Matrix after sorting in an array` `def` `findKthElement(N, M, K):` ` ` `# Initialize low and high` ` ` `low ` `=` `1` ` ` `high ` `=` `N ` `*` `M` ` ` `# Perform binary search` ` ` `while` `(low <` `=` `high):` ` ` `# Find the mid` ` ` `mid ` `=` `low ` `+` `(high ` `-` `low) ` `/` `/` `2` ` ` `# Check if the count of` ` ` `# elements is less than mid` ` ` `if` `(countLessThanMid(mid, N, M, K)):` ` ` `low ` `=` `mid ` `+` `1` ` ` `else` `:` ` ` `high ` `=` `mid ` `-` `1` ` ` `# Return Kth smallest element` ` ` `# of the matrix` ` ` `return` `high ` `+` `1` `# Driver Code` `if` `__name__ ` `=` `=` `"__main__"` `: ` ` ` `N ` `=` `2` ` ` `M ` `=` `3` ` ` `K ` `=` `5` ` ` `print` `(findKthElement(N, M, K))` ` ` `# This code is contributed by Chitranayal` |

## C#

`// C# program to implement` `// the above approach` `using` `System;` `class` `GFG{` ` ` `// Function that returns true if the` `// count of elements is less than mid` `public` `static` `bool` `countLessThanMid(` `int` `mid, ` `int` `N,` ` ` `int` `M, ` `int` `K)` `{` ` ` ` ` `// To store count of elements` ` ` `// less than mid` ` ` `int` `count = 0;` ` ` `// Loop through each row` ` ` `for` `(` `int` `i = 1;` ` ` `i <= Math.Min(N, mid); ++i)` ` ` `{` ` ` ` ` `// Count elements less than` ` ` `// mid in the ith row` ` ` `count = count + Math.Min(mid / i, M);` ` ` `}` ` ` `if` `(count >= K)` ` ` `return` `false` `;` ` ` `else` ` ` `return` `true` `;` `}` `// Function that returns the Kth` `// smallest element in the NxM` `// Matrix after sorting in an array` `public` `static` `int` `findKthElement(` `int` `N, ` `int` `M,` ` ` `int` `K)` `{` ` ` ` ` `// Initialize low and high` ` ` `int` `low = 1, high = N * M;` ` ` `// Perform binary search` ` ` `while` `(low <= high)` ` ` `{` ` ` ` ` `// Find the mid` ` ` `int` `mid = low + (high - low) / 2;` ` ` `// Check if the count of` ` ` `// elements is less than mid` ` ` `if` `(countLessThanMid(mid, N, M, K))` ` ` `low = mid + 1;` ` ` `else` ` ` `high = mid - 1;` ` ` `}` ` ` `// Return Kth smallest element` ` ` `// of the matrix` ` ` `return` `high + 1;` `}` `// Driver code` `public` `static` `void` `Main(String[] args)` `{` ` ` `int` `N = 2, M = 3;` ` ` `int` `K = 5;` ` ` `Console.WriteLine(findKthElement(N, M, K));` `}` `}` `// This code is contributed by Rajput-Ji` |

## Javascript

`<script>` `// Javascript program for the above approach` `// Function that returns true if the` `// count of elements is less than mid` `function` `countLessThanMid(mid, N, M, K)` `{` ` ` `// To store count of elements` ` ` `// less than mid` ` ` `let count = 0;` ` ` `// Loop through each row` ` ` `for` `(let i = 1;` ` ` `i <= Math.min(N, mid); ++i) {` ` ` `// Count elements less than` ` ` `// mid in the ith row` ` ` `count = count + Math.min(parseInt(mid / i), M);` ` ` `}` ` ` `if` `(count >= K)` ` ` `return` `false` `;` ` ` `else` ` ` `return` `true` `;` `}` `// Function that returns the Kth` `// smallest element in the NxM` `// Matrix after sorting in an array` `function` `findKthElement(N, M, K)` `{` ` ` `// Initialize low and high` ` ` `let low = 1, high = N * M;` ` ` `// Perform binary search` ` ` `while` `(low <= high) {` ` ` `// Find the mid` ` ` `let mid = low + parseInt((high - low) / 2);` ` ` `// Check if the count of` ` ` `// elements is less than mid` ` ` `if` `(countLessThanMid(mid, N, M, K))` ` ` `low = mid + 1;` ` ` `else` ` ` `high = mid - 1;` ` ` `}` ` ` `// Return Kth smallest element` ` ` `// of the matrix` ` ` `return` `high + 1;` `}` `// Driver Code` ` ` `let N = 2, M = 3;` ` ` `let K = 5;` ` ` `document.write(findKthElement(N, M, K));` `</script>` |

**Output: **

4

**Time Complexity:** O(N×log(N×M)) **Auxiliary Space: **O(1)

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