Given two non-negative integers **m** and **k**. The problem is to find the **m-th** smallest number having **k** number of set bits.

**Constraints:** 1 <= m, k.

**Examples:**

Input : m = 4, k = 2 Output : 9 (9)_{10}= (1001)_{2}, it is the4thsmallest number having2set bits. Input : m = 6, k = 4 Output : 39

**Approach:** Following are the steps:

- Find the smallest number having
**k**number of set bits. Let it be**num**, where**num**= (1 << k) – 1. - Loop for
**m-1**times and each time replace**num**with the next higher number than ‘num’ having same number of bits as in ‘num’. Refer this post to find the required next higher number. - Finally return
**num**.

## C++

`// C++ implementation to find the mth smallest ` `// number having k number of set bits ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `typedef` `unsigned ` `int` `uint_t; ` ` ` `// function to find the next higher number ` `// with same number of set bits as in 'x' ` `uint_t nxtHighWithNumOfSetBits(uint_t x) ` `{ ` ` ` `uint_t rightOne; ` ` ` `uint_t nextHigherOneBit; ` ` ` `uint_t rightOnesPattern; ` ` ` ` ` `uint_t next = 0; ` ` ` ` ` `/* the approach is same as dicussed in ` ` ` `*/` ` ` `if` `(x) { ` ` ` `rightOne = x & -(` `signed` `)x; ` ` ` ` ` `nextHigherOneBit = x + rightOne; ` ` ` ` ` `rightOnesPattern = x ^ nextHigherOneBit; ` ` ` ` ` `rightOnesPattern = (rightOnesPattern) / rightOne; ` ` ` ` ` `rightOnesPattern >>= 2; ` ` ` ` ` `next = nextHigherOneBit | rightOnesPattern; ` ` ` `} ` ` ` ` ` `return` `next; ` `} ` ` ` `// function to find the mth smallest number ` `// having k number of set bits ` `int` `mthSmallestWithKSetBits(uint_t m, uint_t k) ` `{ ` ` ` `// smallest number having 'k' ` ` ` `// number of set bits ` ` ` `uint_t num = (1 << k) - 1; ` ` ` ` ` `// finding the mth smallest number ` ` ` `// having k set bits ` ` ` `for` `(` `int` `i = 1; i < m; i++) ` ` ` `num = nxtHighWithNumOfSetBits(num); ` ` ` ` ` `// required number ` ` ` `return` `num; ` `} ` ` ` `// Driver program to test above ` `int` `main() ` `{ ` ` ` `uint_t m = 6, k = 4; ` ` ` `cout << mthSmallestWithKSetBits(m, k); ` ` ` `return` `0; ` `} ` |

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## Python3

`# Python3 implementation to find the mth ` `# smallest number having k number of set bits ` ` ` `# function to find the next higher number ` `# with same number of set bits as in 'x' ` `def` `nxtHighWithNumOfSetBits(x): ` ` ` `rightOne ` `=` `0` ` ` `nextHigherOneBit ` `=` `0` ` ` `rightOnesPattern ` `=` `0` ` ` ` ` `next` `=` `0` ` ` ` ` `""" the approach is same as dicussed in ` ` ` `http:#www.geeksforgeeks.org/next-higher-number-with-same-number-of-set-bits/ ` ` ` `"""` ` ` `if` `(x): ` ` ` `rightOne ` `=` `x & (` `-` `x) ` ` ` `nextHigherOneBit ` `=` `x ` `+` `rightOne ` ` ` ` ` `rightOnesPattern ` `=` `x ^ nextHigherOneBit ` ` ` ` ` `rightOnesPattern ` `=` `(rightOnesPattern) ` `/` `/` `rightOne ` ` ` ` ` `rightOnesPattern >>` `=` `2` ` ` ` ` `next` `=` `nextHigherOneBit | rightOnesPattern ` ` ` ` ` `return` `next` ` ` `# function to find the mth smallest ` `# number having k number of set bits ` `def` `mthSmallestWithKSetBits(m, k): ` ` ` ` ` `# smallest number having 'k' ` ` ` `# number of set bits ` ` ` `num ` `=` `(` `1` `<< k) ` `-` `1` ` ` ` ` `# finding the mth smallest number ` ` ` `# having k set bits ` ` ` `for` `i ` `in` `range` `(` `1` `, m): ` ` ` `num ` `=` `nxtHighWithNumOfSetBits(num) ` ` ` ` ` `# required number ` ` ` `return` `num ` ` ` `# Driver Code ` `if` `__name__ ` `=` `=` `'__main__'` `: ` ` ` `m ` `=` `6` ` ` `k ` `=` `4` ` ` `print` `(mthSmallestWithKSetBits(m, k)) ` ` ` `# This code is contributed by ` `# Shubham Singh(SHUBHAMSINGH10) ` |

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**Output:**

39

**Time Complexity:** O(m)

This article is contributed by **Ayush Jauhari**. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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